I have had this happen twice with my AAA pitcher (He is 2023 Drew Rasmussen). He was at 100%, did not get used waiting for his turn to pitch as a starter, then dipped to 99%, but returned to 100 before his start. He has only 45 RL innings and has used about 15 of them.
After he pitched, he was at 73% and the next day indicator said he would be still at 73%. The next game was played, he didn't pitch and it showed that he had been 71% and would be 73% for the next day. How is he getting more tired by not pitching?