Incorrect allele ("2") in... when trying to merge, merged files

[j-gillespie-dna]

bcftools version 1.21

I have about 230 massive vcfs that I am trying to slap together. I kept crashing everything while merging all 230 at once, but I managed to get them merged, zipped, and indexed in two batches. I then wanted to merge the two merged vcf.gz files. However, I ran into an error:

[E::bcf_calc_ac] Incorrect allele ("2") in CC0037_CC0037_N at chr1:939414

which I thought was interesting as these samples have already been merged just fine? Not sure if merging already merged files is a problem or if some sort of artifacting is happening.