I find this a big problem as well. I usually have my Canon set to manual, but with auto iso. If I'm shooting against the light I use the '*' button to lock the ISO setting from an exposure in the shade, recompose. Quick, easy and simple. Trying to mirror this with my FujiFilm x-T3 doesn't work. The AE-L button doesn't do anything. It's got to be a bug that I would think could be fixed with a firmware update?
I can get around it by setting the ISO manually, but it takes time when you're in a hurry to get the shot.