10 days of solid performance then a sudden drop? That's not creative fatigue. That's either audience saturation or a competitor jumping into the auction.
First thing I'd do: go into your delivery insights and watch the CPM trend over those 10 days. If it climbed steadily, you've hit saturation. If it was flat then spiked overnight? New competitor or Meta shifted the auction floor.
Second: what's your frequency looking like? If you're hitting 3+ on a narrow audience after 10 days, the same people are seeing your ad way too many times. That burns you fast.
Also worth noting: did May 21 line up with any campaign changes? Audience expansion toggle, CBO switch, anything like that. Sometimes it's mechanical, not magical.