There is no systematic bias against tanks. There may be a systematic bias against certain styles of tank play, but there haven’t been many complaints about that lately, so maybe that has been fixed. I’ve written a more complete explainer at How Competitive Skill Rating Works (Season 11).
P.S.
If a GM is smurfing in the lower ranks, he can get more of a stat bonus as dps than tank. This is because it is easier to blow the lid off the stats chart with a dps than a tank. This does not affect players that are at or near their proper rank, such as you or I.