Ok, we are resolving issue, [Ticket#2014042366000345].
What we already know:
In my case, the reson for which after upgrade to ROS 6.11/12 from 5.26 the DHCP server bounded to VLAN interface has stopped working is SWITCH !!
Yes, it is D-LINK DGS-1210-52 with DHCP Screening Feature (which allows only specified DHCP servers on specified ports to send offers). After entering here IP address of Mikrotik DHCP server, everything is working fine.
But, the question is, why DHCP on ROS 5.26 was working without IP address entered on switch.
This could be actually ROS 5.x bug, not 6.x
We will compare the offer packets structure from both ROS versions, and maybe find the answer.