Real world practice button disappeared [not a bug/fixed]
The immersion button has disappeared from my home screen, this looks like a bug. Since it is showing for my Portuguese tree (which is completed) but not for my French tree;
It's not a bug; the button only appears if there are untranslated documents available that our system thinks you're ready to translate. Also we recently made a change where only documents in the "Needs to be translated" state are accessible via this button (before it could also return "Needs to be checked" documents). This may explain why the button disappeared for you.
We'll consider changing this to make it more clear what's going on, for example showing the button as disabled when there are no documents available.