This would drastically happen with my amps since I’d bought my house about 4 years ago. Finally got around to monitoring my wall voltage and realized it would swing about +/- 10 volts through the course of a day. 116-127V. That actually makes a huge difference in the way tube amps sound and feel under the fingers. I ran an isolated circuit to my studio and bought a quality power supply with a voltage regulated output of 120V at all times, and now my tone is completely consistent every time I plug in. Might be worth looking into!
Also worth noting that distorted guitars are one of the harshest sounds you can subject your ears to for extended periods. Especially anything over 85dB. So after a length of playing, your ears are fried and you’re dialing it in to sound good while you’re not hearing properly. Then you come back after your ears have rested and it sounds like shit. Any mix engineer will attest to this.