Replicating Belts seems to change underground belt distances. Most of my belts aren't connected when having these two mods together.

https://mods.factorio.com/mod/longer-belts-redux

LUB (Aligned) sets the length to speed tier * yellow (10 for red belts), but with replicating belts installed they are now 9.

Hmm, I'll check into what's going on with that interaction - it ought to be respecting any other mod's adjustments to the vanilla undergrounds. But the only mod that I tested with that changed those values was Bob's Logistics, Longer Underground might be doing it in a different way. (This mod definitely shouldn't change the values itself, did you maybe install Bob's alongside which stomped on the setting?)

Ok, now I see what's going on. The way LUB Redux scans for belt types to modify gets confused when there's any additional belt types in the game beyond what it expects in the scan logic (several of the bug reports in its discussion section are due to that behavior). On line 15 of data-final-fixes.lua in their mod, it excludes one mod's belt type from the scan so that it won't confuse itself with extra entries in the table. A quick fix would be to extend that blacklist to other modded belt types, a better fix would be to whitelist it to only the vanilla belts that they intended to modify.

(I could also throw a little three-line mod together that'd set the vanilla belts underground distances directly to what LUB does, without any mod interactions, if that'd be useful)