_S_1_M_0_N_ did you perhaps use doorman and incorrectly assigned them permissions with the invite token?
Have you tried signing up yourself to see whether you can reproduce it?
The list of extensions do not point to a specific culprit. So I'm unsure how this could have happened. @SKevo please can you check the logs? I don't know how admin is given, but this might also relate to how freeflarum is provisioning its instances.
