I am thinking along the lines of either foam fabricated crown, or wood that was possibly green and not properly kiln dried, or the painter did not do a good job.
Many painters that paint for builders are required to keep costs soo low that they skimp on quality paint and by pass priming and even water down paint.
Stick a straight pin in the molding and see if it sounds like foam or see how easy it is to impregnate?
If its wood, I would either do it myself or give strick instructions on what paint to use and how to properly prep the job. I would not use the same painter either.
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