Test for errors when leaving unbridged channels

[dalcde]

In particular I have seen the following error in production:

Error: 400 DELETE /channels/<REDACTED>/members/<REDACTED>: {"id":"api.channel.remove_channel_member.type.app_error","message":"Unable to remove user from a channel.","detailed_error":"","request_id":"byq49ee7tt8ammx47u8cf8he8o","status_code":400}

[trekkadiz]

I am having a hard time with this error at the moment.
The channel in question is an unbridged PM channel where nothing was written, yet the error persists and the bridge fails to leave the channel. I tried to close the channel manually several times, but that achieved nothing. Restarting the bridge re-opens it and the error pops up.