Go to "Settings", and then "Accounts". Tap hold on lemmy.fmhy.ml and then click on "Remove instance". Once its been removed, add the lemmy.fmhy.net instance.
Also, make sure that you're running the latest version of Liftoff.
If the above didn't work, then re-install the app and restart your phone.
I have faced the same issue as OP. Clearing all app data (or uninstalling/reinstalling) works to fix it, but I get the impression that OP doesn't want to have to re-add and log back into their multiple lemmy accounts after doing that.
Ideally, liftoff would't go into a state of being nearly unusable when your default instance is having problems. That's the point of the post.