One could consider it a bug, but I am not quite sure. I don't know what the middleware does to the interfaces in the background - especially with the two step "Test --> Save" function now present.
I decided I'd wait for the next major release which will be based on FreeBSD 13. The bridging and the epair (jail virtual interfaces) code has been completely reworked in FreeBSD 13.
But of course feel free to file a bug report yourself.