@filikun it could be an app issue, but also because of how federation works with past posts (kinda badly), they might just not be showing up on this local instance because it typically doesn't import past posts (see: https://github.com/tootsuite/mastodon/issues/34) from before anyone on this instance followed them.

The good news is, future posts should show up normally now that someone on this instance is following them. If you want to see their past posts you need to go to https://mastodon.social/@protonmail and interact there.