I cant tell if that ''bug'' is something out there for longer.
So I tried to disable the all chat in TFT(because of the normal LoL toxicity which annoyed me)but it didint work. Probably because they're in your ''team''. Anyway after some days I visited the good old SR again(today) and saw someone from the enemy team talking in all chat. I have it always deactivated there, so I was suprised to see their msgs. I checked the options but it was checked. So i unchecked it, saved and checked it again. But i was still able to see the enemy msgs.