A user of one of my plugins stumbled across this issue:

He somehow ended up with potion items, which seem the same to the clients, but internally, one item has the potion id: 'minecraft:strong_strength' and the other one has the potion id: 'strong_strength'. Otherwise the 2 potion items are similar. This can cause issues when those items are compared by plugins, or by minecraft.

Unfortunely I don't know how he created those items, so I don't know if this is maybe an issue where he created one potion item before minecraft had namespaces, or if he simply missed specifying the namespace when creating the item via command.

If this might be an issue with 'legacy' items, would it be possible to add a conversion to the nms ItemStack class (here: https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/nms-patches/ItemStack.patch#54) to add the missing 'minecraft:' namespace to potion ids?