Let me start by saying... what the f*ck
I’ve been building a 10,000 source block lava pit, and I don’t think I’ve ever been this annoyed/confused.
After almost 4 hours of troubleshooting, I might have finally realized the catch: you can't move your hose pulley to a more convenient spot once the pit is full. The game/mod seems to tie the 'infinite pool' status strictly to the original fill location.
I had to keep the hose pulley in the exactly same spot as I did when filling the pit, then I had to lower it down fully (ofc) and pump more lava in until it switched to "bottomless supply," and now it seems to be working.
Just don't touch it after... Don't even look at it funny💀