We have been having the same issues with 3.15. We have around 3,000 uses whose BW we manage through using PCQ for both upload and download. When we used to be on 3.6 or 3.7 version, the queues worked pretty fine, and all users would get the proper bandwidth.
However, as soon as we migrated to 3.15, we were facing problems. Users would complain that they are not getting their bandwidth. A Torch on the 3.15 router would show that the users were using the full limits assigned. But, that users is actually behind another MT using 3.7 device which does not do any natting, just simple routing. Checking the Torch on the interface of the 2nd MT that the user is connected on shows that the user is getting what the user is claiming, that is less BW.
Another interesting thing to note is, during these times, the Interface would show abnormal usage, say instead of regular 40Mbps, it would show 65 or 70 Mbps. Whereas, on the Queues, it would show only 10 or 11 Mbps. Sometimes, the data would be reversed, meaning more on Queues, but less on Interface.
Is there any fix to this? Mikrotik?