Paint over Bondo

So I've restarted the restoration of my 70 Dart Swinger. I had a door to replace the one on the car and started to sand it. I noticed some paint that was cracked. After sanding done I realized the paint cracked over some Bondo work. My question is was that a result of poor Bondo work or improper prep for paint? I want to be sure if I prep and paint over this Bondo, I won't have the same problem. Thank you.

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