Just adding to what David has posted. The problem is specifically with fields that store values as arrays that get killed. The reason this happens is that wp_kses_post() expects a string input and returns a string as output.
The fields that can cause problems are
Any field that allows the selection of multiple values, like a relationship field, a post object field set to allow multiple, a taxonomy field set to allow multiple, etc.
I’ve been looking and It does not appear that there’s anything that will work on arrays.
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