If you’re sticking tape directly to the mirror, it’s likely depositing burned crud on the coating.
My machine has circular apertures perpendicular to the beamline, so I can tape paper targets on them:
Since that picture I’ve switched to slightly fancier targets with ears:
Other folks use really fancy target holders, so there’s a time for every style.
Switching to molybdenum mirrors, despite their slightly lower initial reflectivity, may finesse the problem by being indestructable:
Just don’t use sandpaper and it’s All Good™. 