Your Question

How does Lumisene determine whether a block should be broken by flowing into it?

Detail

This might double as a bug report but i don't know exactly whats going on. Custom Machinery machines will break when Lumisene tries to flow into them. This doesn't happen with other blocks as far as I've tested, but the machines will also not break if another modded (ordinary, non-finite) fluid flows into them.

Should be the same as normal fluid. What they check wasn't obvious I think it was something with their solid status or a dedicated can be broken by liquid property

If this also works they same with flowing fluid then it's very obviously not a lumisene issue

No, flowing water and lava do not break these blocks. I'll find a mod that uses a forge fluid block to double check if other fluids do.

I'm not sure if Neoforge still offers custom fluid block implementations anymore like forge did, it's been a while, seems like everybody sorta inherits from BlockLiquid nowadays. In any case, sample size increased, Pneumaticcraft fluids also do not break Custom Machinery blocks.

to answer you properly they use a copy pasted method that FlowingFluid uses. This one. They dont call it directly as they dont haev it as they are not a FlowingFluid. Not the best in terms of compatibility tbh but that will catch all cases vanilla has. Also the main property that checks is the blocksMotion property which for solid blocks should 100% be set to true. the only thing it wont catch is mixins into the flowing fluid method

Fixed on Custom Machinery's side