odd merge behavior with 'preserve' keyword and missing some sources in merged output for specific edges

[realmarcin]

I have these two edges in a single transform:
assay:API_20A_ARA biolink:assesses NCBITaxon:818 NCIT:C153110 bacdive:1599

assay:API_20A_ARA biolink:assesses NCBITaxon:818 NCIT:C153110 bacdive:1600
so only difference is the source in the last column. I think this should be merged and listed with ‘preserve’ keyword.
However, in the resulting merge we just have the first edge and not the second and no list of sources:
urn:uuid:a2779de5-8276-4fc3-a6f1-ed81c4285dcc assay:API_20A_ARA biolink:assesses NCBITaxon:818 NCIT:C153110 Graph bacdive:1599
Even more oddly, we have other multi-edges from the same source and these are getting merged correctly with list in source column:
urn:uuid:8844e967-441d-4107-82ec-e38c75590c8d NCBITaxon:239 biolink:occurs_in medium:830 BAO:0002924 Graph ['bacdive:131721', 'bacdive:157661']
(edited)

1:11
The edges getting merged there are :
NCBITaxon:239 biolink:occurs_in medium:830 BAO:0002924 bacdive:131721

NCBITaxon:239 biolink:occurs_in medium:830 BAO:0002924 bacdive:157661
1:11
So structurally no different then the assay edges above? Hmm …