Preview no longer annotates images with shapes?

I often annotate screenshots with rectangles and arrows etc. when asking or replying to questions on the FCP forum.

Suddenly (a few minutes ago) Preview stopped me annotating with shapes . . . text still appears to be working.

All that happens with shapes is that I get a dot at each corner of a bounding box.

I also found that when closing an open .jpg I got some garbled numbers and letters added to the name of the image.

There have been no recent changes or updates to my iMac.

I have tried different images and restarted my iMac but the problem persists. Any ideas?