GridBox properties - not reading saved values - caused hours of lost work

Hi - my current set up is:
Joomla 6 with latest GB, Firefox

I need to update the font sizes on all my websites. This morning, I opened a website, made changes to the Theme-> font sizes and the font sizes to all the elements throughout the website.

I saved regularly, testing on that the changes were working.

However, when a page is closed and then re-opened, the GB property editor frequently shows the old values. Consequently, if saved, all the previous work is lost.

I know the changes were successfully saved, as the live pages (after refreshing) show the font changes.

Please see attached images. File minimization was off. I do not use a CDN. I logged into Joomla afresh (to guarantee the session was not stale). Change H1 to REM, saved, checked the live site, logged out of Joomla, logged into Joomla, and looked at the H1 setting - the previous value has returned, despite the live site being correct?

This morning I carried out many hours of work, logged back in after a break, made one change, and overwrote all my previous work. This includes not just the Theme properties, but other individual components also (e.g. testimonial slider, icon list etc)

I'm confident I had already updated this website a few week's ago. So this issue existed before the update to Joomla 6 + latest GB update.

I will try to update another website later to see if this is site specific.

Can you please take a look, thanks,
Marc