Basically my setup is

Firestick 4k connected to 1st HDMI port with my soundbar to the 2nd port (2nd port also supports earc). I basically get a AV synch issue every now and then and I have no way to solve it, even resetting the TV doesn't work. It goes away later and comes again. The soundbar was connected to my old TV for 6-7 years and I never had any issues.

So I tried disconnecting the soundbar and yes the issue still prevails with TV's internal speaker without firestick connected, so the culprit is definitely the television set. Help me out!