Do u use a custom theme, or are you using the default responsive theme without any modifications?
It happens with both my custom theme (based on Responsive) and Responsive itself. I checked the Reponsive and Bare frontend theme directories against the 5.51 commit and they are indentical.
In terms of plugins, there are very few installed. And only two that affect the frontend in anyway. I won’t name them at the moment because it may not have anything to do with it, but I disabled one about 45 minutes ago and so far I’ve not seen the problem. I need to give it a bit more time though.
I’ll update as I know more. Thanks.