I have this problem on my second NIC. There is an error message that the driver loses the carier.
I added a full restart of the bridge interface at wake-up time in the parameters of the plugin.
In "custom commands after wake-up":
/etc/rc.d/rc.inet1 br1_restart
It may not be ideal but at least, it works everytime.
I had the same issue.
Upgrade to 6.9.0 (and 6.9.1 for that matter) and parity was checked at each reboot.
I applied the 100 sec shutdown timeout and it is ok now.