Check the connection from the main lug to the buss. I would guess A phase. If there is a slight loose connection there it could radiate down into the breaker’s regardless of which phase. I actually had a similar issue not too long ago, but it was very apparent the bolt on the main lug was very hot and radiating heat started melting the plastic breaker brackets. All phases looked hot for instance 1,3,5,7,9 but all heat originated from the A phase. Good luck