What's your perception like? I used the See Invisibility spell rather than Volo's Eye or a potion, but I could see most invisible enemies - not all! If you're not seeing some, that's deliberate but you might want to give the potions to someone with higher perception if it's low. If you're not seeing *any* invisible enemies (and they're definitely there), then that's probably a bug.
It is a bug. I have the very same one. I also can't turn invisible from any item effects, spells or racials, only invisibility potions. I get around it by inflicting the wet effect on enemies, which spreads the same bug to them. To be precise, if there's anything resembling a status effect active (temporary HP, arcane synergy, etc), it knocks me (and enemies) out of invisibility within 0.1 to 2 seconds. The offending status effect will flash in white text, and below it invisible will appear in crossed out white text. Depending on build, something else might work better. I liked wet because it stays around for 3 turns and my AoE focus was lightning.
