@sgunhouse I've seen that setting and it's currently off. Even when I tried to restart my Mac, Opera is the only one that doesn't close. It started happening around the beginning of this year...

Same here. Just updated to Version:78.0.4093.112 (x86_64) and seems to be working fine now. I noticed it happens a lot after a full day of using Opera too. I can't even close Opera without using Force Quit.