Paint Bucket does the wrong color

I'm working on a Macbook 2016, latest OS Catalina, with Photoshop 21.1.1.

I can't control the color the bucket tool lays down, on the most basic level:  new project, original bottom layer--automatically blue. I pick another color (pea green) from the two color boxes at the bottom of the tool bar > bucket ADDS that color to the blue, making a very pale slightly greenish color.  Click again > goes back to blue. Click multiple times, it oscillates between blue and increasingly saturadated pea green, until finally reaching something more or less the pea green I selected. Try a different color (deep red) > same deal, gradually shifting the color towards red, but now alternating with black.

It's as though I have the blend mode set on Difference. Blend mode is NORMAL, opacity is 100%.

I try it with a new empty layer > same weirdness. I try filling in a Marquee selection > ditto. I draw a rectangle > same deal.

A new Fill or Gradient layer from the Layer menu finally behaves.  So I do have a workaround. But I've been doing the previous steps without problems for years--what's up? I first encountered this problem yesterday when I was online in Zoom, trying to teach the basics to students! Major egg on my face--and one of the students was getting the results I was after on his computer, no problem.