If you google something like:
google console reporting mobile issues when none exist you will see many reports of the issue – none of which are fully explained aside from Googles console tests being a little flaky.
As there are no visible issues on your site and it passes the mobile friendly test we can go with it being google having a weird moment.
I wrote this doc a while ago – which is more relevant when Mobile Friendly tests fail:
Gives you a little understanding of why a test may fail. Releveant takeaway point is that Googles resources fluctuate, and when they’re really busy that simply can’t load all the necessary JS/CSS files to render the site properly.