Thank you for your response and for taking the time to explain.
However, in my case I don’t think this rule applies the way you describe. The money I played with was from the free spins, yes — but the casino required me to wager the winnings 50 times before any withdrawal was even possible. So I couldn’t withdraw anything at that stage, even if I wanted to.
While completing the wagering, I won over $6000 and the balance was fully credited to my account. I continued playing for quite some time without any warning or restriction from the casino. Then, suddenly, the entire balance disappeared.
If the rule was that winnings over $100 from free spins are invalid, they should have blocked or capped the balance automatically — not allowed me to wager and play normally for days and then remove everything later.
To me, this looks like a failure on the casino’s side, not mine. Players can only follow the written rules and the system in front of them.