If you smash the same blocks you don’t get xp. My guess is it saves the last 100 or so block you destroyed. So smashing different blocks allows gaining xp again. A lot of the orb farms are small so you can’t do this.
I think if you regen and destroy block too fast it wont give xp also.
Try it at a slower pace to see if you gain xp back. There was a similar issue months ago with someone else. He was regen farming with full speed and gain no xp, i tried it without having a speed tool and i was getting xp for every blocks.