With a 2.4.0RC3 installed from scratch, the thumbnails are not in cache : they are regenerated each time I refresh the thumbnails page. As soon as I validate screen [Administration > Configuration > Options > Multiple Size] the cache is correctly used.
I believe this is since the "Restore default settings" feature - because last_mod_time of original settings gets updated on every page call.
A fix would be in ImageStdParams to call save() from load_from_db() just after make_default()
BTF, off topic - I saw several issues with external imagick - I believe one is the rotation which behaves badly ... (no tests but I think we miss a switch between this->height and this->width in class image_ext_imagick, function rotate if rotation is 90 or 270 ...)