It doesn't mean they did that lawfully. Anyway, I don't know what exactly you told them, how you answered their questions. I've been questioned many times, and always allowed in.
I've been in Thailand as a tourist since the beginning of 2020, never leaving for more than a few days. As long as I don't stay longer than each stamp allows, it's not a problem.
This is how it is. Russians CAN do unlimited land border bounces, but they CANNOT get 30-day extensions on visa exempt. It is a different bilateral agreement between Thailand and Russia, not like with other Western visa exempt countries.