The official Mastodon apps still do not indicate if an account has transferred to a new server. (The website, web app and third party apps do indicate this correctly.)
This means people using Mastodon through the official apps may attempt to follow the old accounts and just get an error message, without knowing why.
If you're comfortable using Github, you can vote for this to be fixed by giving thumbs up at these issues:
Android App
https://github.com/mastodon/mastodon-android/issues/415
@FediTips The web app oddly enough has more features than official Mastodon app.
It's because the web app has been around since 2016. The third party apps have been around for many years too. They're mature and full of features.
The official app store apps were only introduced in 2022 and are still missing lots of features. They were mainly released as new people were so used to Twitter, Facebook etc being so oriented around official app store apps, and assumed that was the only way to sign up.