Sudden Mobile Usability Issues with GP

thats odd....
I ran a Mobile Friendly test which failed - looking at the HTML tab in the test report starting on line 35 i see a huge inline CSS file.

However when i inspect the actual site in my browser that inline CSS is not there.

I am not certain but maybe a server level cache or CDN ( e.g cloudflare ) is serving up a different page. Check with your host to see if they can explain the difference.

Got stuck in a circular conversation with Bluehost, ended up suggesting I needed to use a plugin on my site to make it mobile compatible! We know GP is a responsive theme! They don't seem to have any idea. So I deactivated both W3 Total Cache and WpSpeedup plugins now and got them to clear the cache from the server side again. I also purged the cache from Cloudflare. Looks like it may have fixed it?

Thats fixed the issue.
Sounds like Cloudflare cached an older copy of the site.

BTW is it the same thing with the possibility of being given false readings (like you said can happen with the Mobile Usability test) if you run Page Speed Insights tests too many times close together?

No - this was an issue with Google being served what i can only assume was an incorrect cached page.

Sorry David, I worded my question badly: I meant, can Google Page Speed Insights return false negatives if you run it too many times in a row, like you said can happen with the Mobile Friendly test?

And does false negative in this case mean a) it's likely to tell you there's a problem when things are actually fine... or b) it's likely to say things are fine when there's actually a problem?

If you see what I mean?

The false negatives generally report the Other Error in the Issues - see here:

https://docs.generatepress.com/article/text-too-small-to-read-and-clickable-elements-too-close-together/#other-error

Google won't ever report a false positive.