is there any kind of premium matchmaking?

[Gazzza]

ive bought a chunk of premiums vehicles and have noticed im getting downtiered way more often,

coicendence or is somethin going on

[Eftwyrd]

there is definatly no premium matchmaking, it is all chance and a certain amount of favouritism from the playerbase as some BR brackets are more popular than others and all other Brs in range tend to get dragged into them, in ground for example its 3.7, 5.7, 6.7 and 8.0, if your linup is in range of any of these you are more likely to get matched with them than anything else.

[Serpent90]

Matchmaking is dependent on where most players are.
If a new popular tank is introduced at 5.3 br, you'll likely be uptiered when at 4.3 and downtiered at 6.3. It keeps fluctuating for a gazillion reasons.
If you notice that you get uptiered a lot, try playing at your current br +2. Or switching to a lower one.

[Illusionyary]

I can assure you this is not a thing. My Piger (a 6.0) gets thrown into 6.7 games all the time. I'm lucky to get something that isn't a 6.7 (or even worse, a 7.0).

[Strez]

Certain BRs are a lot more popular. So 5.3/5.7 often get uptiered to 6.3/6.7 in tanks, for example.