https://wiki.project1999.com/Buff_Lines#HP_Regeneration - This page has more stacking info than anyone needs
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Long story short: different regen types (see page) do stack, but regen caps at 15 .. so stacking doesn't matter once you wear a fungi: you're capped. It only matters for (say) Iki BP + something else.
However Aura of Battle is not considered regen, even though part of what it adds is 2 (I think) regen. Multiple AoB should stack, but it doesn't here (unsure if that's timeline appropriate or just P99 "selectively classic").
Adding it all up 15 (from Fungi or a combo of other items) + 2 from AoB is the true max ... and I think the belt you mentioned is AoB so it should stack.