I can confirm that the issue does not lie with SFML.
The max antialiasing level on M1 chips is 4, and my code was set to use 8. When I changed it to 4, all RenderTextures worked again (still compiled for x86_64 and run through Rosetta).
I went a step further and compiled my project for arm64 and it runs natively on the M1 flawlessly.