A quick word at the start: Ofc pylons isn't the only mod I use on my server.

The modpack has been custom compiled by me from scratch and (aside from some normal hiccups) had been working mostly fine. I don't want to send you down any rabbit hole you don't want to dig yourself into, but if you intend to replicate the "bug" fully and completely, I'd reccomend you download the modpack and just try it in the "original condition". (Technic launcher: "Krisuun_client" - 1.20.1 *cough *embarassing)

If you don't want to do that, I'll attach a modlist.txt to this issue post that you could quickly scan through, maybe spot mods which you already know to clash with your mod. Just note that it is the "client side" mod list. If you need the server side, let me know. modliste.txt
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Setup for the bug/ glitch:

1. Go to the end and move into the 1th void ring (/tp 0 100 -450000) build a small platform and place the interdiction pylon underneath.
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   BTW: The black areas on the minimap are building parts and are partially lit, so enderman don't spawn there very often.
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2. Fill in a mob filter "containing" the enderman. Extend the radius to 5x5 chunks, press "on" ... and be amazed about the still spawning enderman, which are clearly unimpressed by the pylon (^_^). Yes I did reload the area after enabling the pylons. They still spawn uninpeded.

PS: The lifeless filter does not yet exsists in this mod version, right?
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Soooo ... can you help me?

Even should you fail, I'd be interested in the exact method you use to spawnproof the chunks. Does the interdiction pylon change all light levels in the range to at least 1, or how is it done? Once I know that, I could go on the ardous task of trying different mod(pack) variations myself too.
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Yet, maybe I am lucky!? :D
(base project would be costly to abandon - Replica of Jeracraft's tower)

The method I use hooks into the Forge "entity added to level" event. Every time an entity is added to the world, it runs that event and allows mods to block the spawn. I then check the mob type and position against a list of active pylons, and if there are any matches, it blocks the spawn.

In theory it should be very hard for other mods to accidentally break that method but it does work in ATM9 so that's my theory at the moment. I didn't test that far out in the end, but I don't know any reason that would affect it. I'll try your pack and see if I can replicate the issue there.

I loaded up your pack and built a platform at those coords in the end. Waited for the platform to spawn endermen, added enderman and baby enderman mob filters to the pylon, moved 128 blocks away to despawn the mobs, then moved back. Only thing that spawned was an apoth shulker boss, no endermen. Tested with enderman spawn egg to double check and those are successfully blocked. So in your base pack, the pylon seems to be working perfectly.

Can you replicate the issue if you make a new world and test there? You can also use spawn eggs to easily test if it's working in the overworld.

I think I understand my problem ... and it's exceptionally silly.

I created a new world, placed the pylon and it did block any spawns of the filtered mobs. I then tried it in the end and it did work there too.

Then I went back on my server and looked at the area shown in the screenshots again and did some tests there too. My result? The pylon works ... just not the way I thought it would.

Here is how I thought it would work: I thought that the "5x5" was an expression of an radius around the pylon. I guess I was kind of programmed by mekanism, which is expressing everything in a "radius" fashion. I remember thinking "this is kind of big" when I first placed them, yet the harvester und potion pylons are also awesome so I wasted no further thoughts on it.

Reality, however, looks like this, right? It's a 5x5 diameter, right? The only trouble I have, is determining the "short sides" (aka the 2 chunk sides) without relying on spawn eggs.

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It's so silly ... and I am quite embarassed to have wasted your time. (￢_￢;)
I'm even a bit afraid to look on the official mod description page now. Please tell me that the word "diameter" isn't listed there! xD
If it is not, you may consider adding it. It might prevent another fool from being played by himself. Hahaha.
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Result? The user is stupid, the code, however, Sir, is without issue! xD
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Thank you a lot for your active support and your overall time expenditure! That isn't a given! Thanks a lot!

That's probably my fault for the confusion, I do use the word radius in the tooltips and in the JEI info page because that's what the code uses under the hood, but what you actually see in the UI is the size of the area not the radius.

So it would be a 5 chunk x 5 chunk square centered on the chunk the pylon is in, this aligns to chunk borders so exact placement of the block within the chunk doesn't matter. You can see the chunk borders with F3 + G.

I think, I figured out the "short sides" too! => There are no "short sides". User error again. :D

1 Pylon in the middle and then 2 chunks in every direction from that middle chunk.

Doing even simple math while being ill seems to be an impossible challenge. xD

(I think we can leave it like that, right? Thanks a lot again, m8!)