I can’t reproduce it on iPhone 8 under identical versions of iOS and Joplin. Everything runs easy after sharing a website or a foto from Safari or iMessage to Joplin.
It's stably reproduced on my Xs (I tried to navigate back with on-screen gestures and the left-corner link). One more thing, the 5th step is an excess one, the bug is reproduced even without it. Probaly the 3rd-party application is not fully frozen, because I've noticed that video-preview still works on Youtube, but it's impossible to scroll or tap controls on the 3rd-party application.