As it turned out, simple queues on v6 work slightly differently than on v7. Maybe because of a design change, maybe because v6 did not work as intended and v7 fixed that. It took a "bit" of manual diving to find the solution.
The first clue is here:
https://help.mikrotik.com/docs/display/ ... +Switching. Under bridge settings it says:
use-ip-firewall (yes | no; Default: no) Force bridged traffic to also be processed by prerouting, forward, and postrouting sections of IP routing (see more details on Packet Flow article). This does not apply to routed traffic. This property is required in case you want to assign Simple Queues or global Queue Tree to traffic in a bridge. Property use-ip-firewall-for-vlan is required in case bridge vlan-filtering is used.
Below the list of settings is a note that clarifies a bit more.
In case you want to assign Simple Queues or global Queue Trees to traffic that is being forwarded by a bridge, then you need to enable the use-ip-firewall property. Without using this property the bridge traffic will never reach the postrouting chain, Simple Queues and global Queue Trees are working in the postrouting chain. To assign Simple Queues or global Queue Trees for VLAN or PPPoE traffic in a bridge you should enable appropriate properties as well.
On v6, this setting is also disabled by default, but for some reason traffic is being passed to the simple queue anyway. This explains why it stops working after the upgrade. After enabling the setting, the queue is applied as expected.
What is a bit worrying is that I also raised a ticket with Mikrotik support and the response I have had sofar does not point to this explanation and gives a completely different solution that does not work and is simply contradicting the manual and real life experience.
It was said that simple queue is designed to work with addresses, not interfaces. Which is clearly wrong, because webfig happily accepts interfaces and does not flag the queue as invalid. Also, I am using it with interfaces right now and it works just fine now that I have enabled use-ip-firewall.