Rectangular artifact in display Macbook Pro 16 2019

[northernlight]

Had this issue in the past sometime but haven't found an explanation yet. There's a persistent rectangular artefact on the bottom left of the screen on Desktop 1 but not on Desktop 2 (see picture).
When Desktop 1 is deleted, it moves to Desktop 2. SMC and NVRAM were reset a few days before this appeared. After resetting NVRAM it has disappeared. Not sure what causes it but this is strange (and annoying).
Also not sure if connected, but sometimes the speed of text appearing on screen slows down on this machine.

 

[StumpyBloke]

Please post a proper image, rather than a pdf

 

[northernlight]

Please post a proper image, rather than a pdf

Apologies - changed now

 

[Fishrrman]

Is that there all the time?
Or... just some of the time?
If... just some of the time... when....?

 

[northernlight]

Is that there all the time?
Or... just some of the time?
If... just some of the time... when....?

Not all the time, but when persistent when it appears. Reappeared after upgrade to 11.5 etc

 

[Rodan52]

Does it appear in the same position on an Application window on desktop 1?