I am running v6.36 on a RB SXT 5HnD and noticed that the timeout values for connections in IPv6/firewall/connections increase rather than decrease. So the timeout timers will never decrement to 0 for IPv6 connections.
The timeout values for IPv4 connections in IPv4/firewall/connections do decrease.
I am running v6.15 on another Mikrotik box and it appears that timeout values for connections in IPv6/firewall/connections decrease on that box. Not sure if it is a bug in v6.36 or a bug specific to RB SXT 5HnD.