I understand this is something that you might not be able to help with, but I’m stumped.
This morning I got a warnings email from Google Search Console saying there are 3 mobile usability errors with my Generate Press theme and I really have no idea why. These are the errors – https://d.pr/i/EgzfyS
if there is nothing fundamentally different on that post to any other, then the chances are its googles resource limits.
Google caps the server resources it applies when testing sites. Not sure what the cap is and whether it be variable or not. But on occasion it will simply not have enough to load some of the site resources such as CSS and JS. This in turn leads to the errors you see in your report.
If it happens regularly then look reducing the number of requests the site ( or that post ) is making.