I was working over the last week with 2020.5.7 with no problems. Today the clone brush #1 began to fail to set the clone source offset. When I try to set it with my wacom right click button, it just jumps around randomly. When I try to set the offset to zero manually in the paint controls window, the numbers change but the offset does not. Shift+click with the mouse also has the same problem.
My workaround is to use the arrow keys as this is working to set the offset. For some reason, the brush #2 does not seem to be affected by the bug.