My friend Mrmadmax23 is able to use a Invisi Pot without purchasing the Investigator perk. He says he hasn't bought anything on the server.
This is not a bug, but intentional. Those with the Investigator donation feature are the only ones able to use invisibility.
@KMaxwell, I think that's why this is a bug report. This appears to be the same issue as https://www.ecocitycraft.com/forum/threads/161204, which seems to have been fixed back in 2016.