This is the first and only time encountering this issue, and at present I'm not entirely certain what's causing it.

I accidentally teleported to an unpowered, already-deconstructed dimension, and within seconds the game appeared to freeze. The odd thing though is that it does not appear to be actually frozen; the game behaves as though its stuck processing something heavy, and ticks every once in a while. My phase generator appears to drain at the expected rate, which I'm guessing is a result of the "time since last tick calculation", so I can tell it is at least occasionally updating, but inputs like mouse movement and keystrokes are often lost, especially if there's more than one.

I am using a modified FTB Infinity 1.12.1 with a few additional mods included (Galacticicraft, mekanism, meteorcraft).

The dimlets included, to the best of my recollection, were: liquid steam, biome deep ocean, terrain flat, time morning, sky bright day, and possibly the McJty dimlet. (I realize the flat terrain screws up the liquid; this was one of my many experiments as I've only recently started playing around with this mod.)

I can provide more information, or the world itself if needed, later this evening. Let me know if there's any additional information required (or whether I'm just doing something boneheaded).

This kind of report is nearly impossible for me to debug :-/
It certainly does not occur in all dimensions though

Thats correct, it does not occur on all dimensions. This is the first time encountering this.

I am a developer too; I agree this is not a lot to go on. I will attempt to dig deeper and see if I can find some kind of more specific information.

If you ever have an update, please do reopen 👍