Ah sorry. I searched the code using the term you posted. I see it now with the full link text.
So the second question was about the forum menu which – to save going around the loop again – I am going to assume does NOT give you the single option to update and still shows in the toolbox as being at 5.5.2.
The ONLY inference I can take from all of this is that WordPress is failing to actually perform the download and replacement of the plugin code even though it is reporting that it has been successful in doing so. Would you agree that that is about the only realistic conclusion we can come to?
And if that is the case – and I can’t begin to tell you what may have caused WordPress to misbehave in this way – then the only reliable recourse would be to perform a manual upgrade as described here: https://simple-press.com/documentation/codex/installation/upgrading/upgrading-version-5/
Is that going to be possible?