Patchy first layer on glass bed

The part at the front has only one layer, and has what appears to be uneven patches. The part at the back has had two layers, so it clears itself up.

The build plate is flat, and level (I levelled it before the print) and by the way the patches are all together I think I can rule out flow issues.

What is going on, and is it a problem? See the image below, for an example:

Setup and configuration:

• Sheet glass bed
• White PLA (Geeetech from Amazon) – opened the same day
• 190 °C nozzle, 55 °C bed
• Ender 5 printer, with Cura 4.6 slicer

I have changed the filament (still PLA), re-levelled the bed, and here is the bottom of a large flat surface:

Also the top surface has ‘ringing’ artefacts (I think that is what they are called?):