So how are some supermarkets advertising on packages claiming that fresh produce (e.g. Broccoli) contains synthetic folic acid?

I can’t answer that question from the retrieved records. The evidence block contains exactly one record, and it is about encapsulating 5-MTHF in a pectin-alginate matrix for food processing, not about spraying crops or post-harvest treatment of produce [1]. No retrieved record describes folic acid being applied to fresh vegetables, no record mentions shelf life, and no record supports the claim as stated.

The specific sub-question that would be answerable with better retrieval is whether any regulatory or industry document describes post-harvest application of synthetic folic acid to produce. That record does not exist here. The retrieval is thin, and the claim is unsupported by anything retrieved.