This took a good day or two to narrow down but eventially I found that simply having a set of relays go from a hard cable network in a force-loaded claimed chunk (from FTBChunks), outwards to non-forced non-claimed chunks, would result in the server locking up on closure, in particular when running a singleplayer world, it would just get stuck on "saving world" with no log messages and staying there pernamently.

It didnt matter whether these chunks were already open by the player at the time of closure, and surprisingly nor does unclaiming the chunks result in the issue going away, it just stays regardless after being initially triggered and about the only thing that can be done is to remove all of the wires and hope that works, or just restore the world from a prior backup before the wires were placed.

corrected that I mistakenly put 1.20.1 in the title instead of 21

also it seems to be specific to just integrated dynamics wires, and each end in each of the force loaded chunk ends just needs a single ID cable without any attachments besides the flying wires themselves, why exactly isnt clear...

also further; it seems like to create the issue it needs to specifically be in a "standard" world, not the default green grass flat world, creative mode doesnt matter but it seems likely that the issue occurs specifically for networks in the positive Y range, and/or with networks that cross negative and positive X/Z axis

but still, AE2 networks in the exact same places are unaffected, but not sure about the CC ones as I havnt yet used any

interesting extra behavior; if a AE2 wire is run alongside the ID one, forming a valid end to end connection, this seems to stop the server hang from happening, but once you remove one end of the AE2 network the hang will occur again