There’s some detail missing from your description, but it sounds like your field (not column) began as something like a formatted date. You then changed the field to be a link to another table, and in building that table, Airtable changed the format. Is that correct?
Here’s what’s happening: when you convert a field from a manual entry type of any kind (text, number, date, etc) to a link field pointing to another table, Airtable builds the new table from scratch, and uses the existing data to create that new table’s initial records. In doing so, it’s making brand new fields, which all have their default formatting options. While it might be nice to have the formatting copied to the new table’s primary field options, my gut says that the devs felt it was more important to ensure that the data carried across cleanly, not necessarily the formatting.
That said, you can still return to the formatting you want by editing the formatting for the new table’s primary field. Whatever format you apply there will appear in any other fields that link to those records.
If you feel it’s important to maintain the formatting when converting a field from manual to link, I suggest adding a post to the Product Suggestions category.