This bug happened rather a while ago back in my days in 1.11 when I was trying to punish someone on my realm. For that reason I am not sure whether it is relevant now in 1.12.
How It happened/ how to reproduce: I had some problems with annoying player, and to punish them I used a /gamemode 2 and /tp device it accomplished what I needed,yet I had not expected what I will mention below.
The unexpected: When I turned it off and they came back on they couldn't move, until I banned them and put them back on.
How to resolve: Ban the player and re invite them.
Please check if this is still an issue.