I think it a bug, I get this problem too.
I'll be in defensive mode & I get a red message saying Defensive disable. I had to wait 5 mins to change back to defensive mode.
The way I fix this, so the game stop disabling defensive mode.
When I enter a server I'll be in defensive mode. I will switch defensive mode off & back on seems to fix that issue. As long as I stay in the same server. If I switch server I could encounter that problem again & would have to switch def mode off & back on again.
Your flag will lower when the defensive disable glitch happens. You will have to manually raise your flag.