To reproduce this bug, open the color sliders (F4), and select a color from your pallette to edit. Go to HSV or HSL sliders. Try inputting, for example, hue at 50, either by manually inputting the number by typing it in the box, or by precisely sliding with your mouse. Now click another color and go back to the first one. It might or might not have subtracted 1 from your original value. It also randomly subtracts 1 from other values which you have not touched. It doesn’t seem to happen for RGB or Grayscale sliders.
See example gif below:
EDIT: on second thought, this might not be a bug, but some sort of web-safe color correction feature. In which case, is there any way to turn it off? It’s incredibly annoying.
EDIT2: Apparently this is a known issue: My HSV color keeps going back to 69 · Issue #2245 · aseprite/aseprite · GitHub