Randomly pixelated layer mask

Sometimes when I create a layer mask on a reasonably decent quality image it becomes really pixelated for no apparent reason.

I am using the web based version cause my computer struggles to run the app if anything else at all is open on my computer and I thought I could do this on the web.

I don't know how clearly you can see, but it selected the object just fine and it wasn't pixelated at all, but when I created the layer mask it was so pixelated. Same story when I used the add to/subtract from mask brush.

I exported the image to see if it was just not displaying it to the highest quality but it was really pixelated there as well.

But wait - it gets weirder (I don't have pics of this cause I deleted what I was working on)

This issue kicked in the other day while I was halfway through something and half of the mask was a normal level of pixelisation ie. as pixelated as the image, but part of it was really pixelated like the above image.

But essentially (in both cases), I selected the bit I wanted (all good) then I clicked the "create mask" button (all bad) and I don't understand why or how to fix it