Reproduction
1. Open a WEBP, which has the sRGB Exif flag set.
2. Editor window has color management ON.
3. Use the color picker to set the foreground color which will be used as the rectangle's fill color to the same color as the area where it should cover something (e.g. text).
3. Create a rectangle (vector or instant bitmap) with a fill color and no stroke over the stuff you want to cover/hide.
5. Your rectangle is now invisible (if vector: selection/grab-points) and can be used to cover stuff (e.g. some text) which you want to hide.
6. In the Editor window bottom action bar toggle color management OFF.
Unexpected behavior: Now the rectangle is visible again! (a different kind of gray).
That's wrong! It should be included in color management and remain invisible.
Workaround:
In step 2: Turn color management OFF. As the rectangle is not color managed at all, at least its raw pixel values are then the same.
If in step 6 you would then toggle ColorSync ON the rectangle again falsely gets visible. Anyhow don't do this.
Remain with ColorSync OFF. Then close the file, click SAVE, and then in the followup dialog agree to FLATTEN the vector objects into the bitmap file (WEBP).