The Feral Flare Lantern has the capability to apparently nonstop grow the amount of light sources it stores.
For context, I placed a lantern near the sky height in the modpack StoneBlock and shortly after doing so nobody was able to go near that chunk without being kicked from the server with a Badly compressed packet exception.

This screenshot is of a program I wrote to determine what the largest TileEntity was in a chunk to help resolve the badly compressed packet error, and it turned up that this lantern was by far the largest block

The light list continuously grows. Infinitely. My theory is that it tries to place blocks above the world height limit and then fails to detect that they are actually there, so tries placing more and repeat cycle. That or the world limit prevents light levels from ever being satisfactory on the topmost block.

Image of setup:

Badly compressed packet exception:

oh.... shit. Yeah, that might be a thing. Another edgecase I've never even thought of. Ty for reporting this, I'll try putting out a fix asap. Btw, which version of the Mod are you using?

Fixed in 1.7.1.74 for 1.12. Should be available soon on curseforge.
This will not fix already buggy worlds since I'm not sure how many people are affected by this issue. Adding a workaround just for this bug will add unnecessary overhead for something that maybe 5 people experience. I suggest removing the lantern and placing it a again (if possible) to reset the internal list.

If you are affected by this exact issue and dont know how to fix it (and you are sure its happening because of the lantern and not some other mod), please report here and I'll consider adding a workaround.

Sorry for any inconvenience this bug has caused.

I just realised.. those project:e watches probably accellerated the issue. How long did you run this setup before you noticed this bug?
I'm considering adding a more sensible hard limit, just in case something else can cause this issue as well.

The block was placed about a day before any issues were noticed, but probably closer to 20 hours before it would actually happen. The version was 1.7.0.72. Also there were 548,072 light entries in that tag, so 255 seems like a more, ah, reasonable number.

Thank you for the quick fix :)

thanks for the detailed response :) I'll close this then.