Intermittent magenta/yellow blotches can come from several places, so the best approach is to isolate variables.

Likely causes mentioned:

• Processing artifacts: check whether it appears in the original file or only after editing/export. RAW vs JPEG, color profile changes, reduced bit depth, or aggressive adjustments can all cause color artifacts.
• Overprocessing: excessive saturation or clipped channels can exaggerate magenta/yellow blotches, especially in skin tones or highlights.
• Lighting/reflections: colored nearby objects, window light, glass reflections, prism-like effects, or even a failing bulb can create odd color casts.
• Filter issues: a dirty, smeared, or poor filter can cause strange color contamination.
• Possible sensor/heat issue: if it happens more after extended use or in warmer conditions, test for overheating.

Useful tests:

1. Photograph an evenly lit white sheet of paper, slightly out of focus.
2. Compare RAW and JPEG results.
3. Check an untouched original versus your edited/exported file.
4. Remove any filter and repeat.
5. Change the light source and location.
6. Note whether the issue appears after the camera has been on for a while or in a warm room.

From the answers, overprocessing/clipping is a strong possibility, but systematic testing is the best way to confirm it.