How to distribute power better across two breaker panels
You did the right thing first tightening the connections, next would be to use an AMP clamp meter to verify the loads connected to L1, L2 and make sure they are ballanced, if unballanced as stated above some breakers need to be moved untill they are ballanced. if they are ballanced it may be your main breaker has a weak hammer on 1 leg causing some internal arcing and the voltage drop. I had to replace a square D panel several years ago as I could not get a new main breaker from square D