<p>Am just wondering if any conclusions can be drawn from this:</p>

<p><a href="https://i.stack.imgur.com/0RNE3.png" rel="nofollow noreferrer" title="Photo of poor adhesion"><img src="https://i.stack.imgur.com/0RNE3.png" alt="Photo of poor adhesion" title="Photo of poor adhesion"></a></p>

<p>Three corners are solid, but not the one in the centre of the plate.</p>

<p>The bed was levelled before printing (and checked afterwards also). Even though the photo may <em>appear</em> to show a slant or lower corner (where the print is coming off), there is not. The bed is level, relative to the extruder, at room temperature.</p>

<p>The temperature of the bed is about 70&nbsp;°C. I get inconsistent readings (with laser thermometer) but to the finger it feels about the same everywhere.</p>

<p>It's a glass bed, presumably with some coating. Is it degraded? Local temperature variation? Any ideas anyone?</p>

From here:
https://io3dprint.com/review-anycubic-i3-mega-ultrabase/
Ultrabase Bed
  The Anycubic i3 Mega Ultrabase is the latest version in the Anycubic i3 family. As hinted in the name, the main upgrade from the previous version is the Ultrabase bed. This is a textured coating on the Borosilicate glass bed that means you don’t need to apply any glue or tape to the bed to make your prints stick to it.
Ultrabase is similar to the popular BuildTak beds except unlike BuildTak it doesn’t wear off and the most significant benefit is that parts are exceptionally easy to remove once the bed has cooled.
The Ultrabase surface has a Moh’s hardness of over 7. This means you can safely use metal scrapers and blades to clean it without risk of it scratching!
Perhaps it was just not cleaned sufficiently from a prior print.