Why is synthetic folic acid a bad thing?

The mandatory addition of synthetic folic acid to our food supply back in the 1990s was in response to an increase in neural tube defects in newborns. Fortificaton of processed foods initially appeared to have a positive effect in reducing this serious birth defect in women with folate deficiencies. Recently published research concluded, "National fortification with folic acid is not associated with a significant decrease in the prevalence of NTDs at the population level." PMID 31963665. In addition, the science is beginning to show that too much synthetic folic acid may be causing unintended consequences. Some segments of the population who use multivitamin supplements in combination with fortified foods, likely far exceed a folic acid intake of 1 mg/d, which is the tolerable upper intake level defined by the Dietary Reference Intakes. Research has shown at or just below a 400-µg dose of synthetic folic acid (the recommended dose to prevent spina bifida), this dose is converted into biologically active methylfolate during absorption. Above this dose, synthetic folic acid is transported in an unmodified form (unmetabolized folic acid/UMFA) into the blood in a manner that is directly proportional to dose, and magnified further by the number of T risk alleles in MTHFR C677T SNP. This last observation likely results from the inhibition of MTHFR by synthetic folic acid.
Folic acid supplementation is implicated in:

• Greater risk of developing multiple, or advanced polyps in those with prior adenomas
• Masks B12 deficiency of the elderly (cognitive decline and anemia) and greatly increased risk of peripheral neuropathy in those with TCN2 snps
• Unstable angina
• Causation for cleft lip/palate
• Increasing rates of various cancers due to epigenetic/DNA methylation changes or reduction in NK cell activity (inconsistent results)
• Non-alcoholic fatty liver disease
• Increasing levels of unmetabolized folic acid (UMFA) with unknown consequences