It appears that not only are tier 8 tanks hampered by the post 9.18 matchmaking and only get to be top tier 15-21% of the time when they were top tier 35-41% of the time pre patch 9.18. In addition to that, there is another matchmaking bug.
The tier 8 solo platoon bug has been confirmed to be on the NA server as well as the EU and RU servers. If you create a platoon, don't invite anyone, then play tier 8 tanks you get tier 8 battles over 50% of the time in tier 8 tank. Some players are reporting that they get tier 8 battles 100% of the time. Post 9.18 tier 8 tanks were getting tier 8 battles about 20% of the time in the 7 PM to 11 PM time frame. The bug more than doubles your chance of getting a tier 8 battle in a tier 8 tank post 9.18 and sometimes gives you all tier 8 battles depending on the server population.
One EU streamer did this for about 4 hours yesterday and got almost all tier 8 battles in his tier 8 tanks.
This bug also seems to work with other tiers with mixed results.
Wargaming management in Mink stated they are aware of the bug and plan to fix it. There is no estimate on the fix time frame though.