It seems like you have empty links where values contain comma (or &).
That's because if you try to fill linked field by copying values, for each value it trying to find first match, and links to it. If nothing found, it creates new empty record with that value and links to it.
That's very important to understand, to avoid wasting tables with strange empty records.
So, if you try to paste value with comma, AT process it as two values, different from value you are trying to link. In other case, it would create new records, but here it can't because it's a synced table.
Workaround? I usually create temporary duplicate field, susbstitute comma with underscore, in both tables, link table by copy-pasting field, revert _ back to , and remove unnecessary fields.
But in general, I'm planning my base in a way to avoid such issues.
