SQL query returns row with empty field for links which are set to display Single Selects

Not sure if should be tagged as a bug, or it’s by design:

When querying for rows with SQL and convert_keys is true, any fields which are links to other rows are returned empty for that row if the column to display is a Single Select.

Example:
Table Assembly has column called PCB, which is a link to a table called PCB.
Table PCB has a column called Type which is a Single Select
The Link column on the Assembly table is set to display the Type field of the PCB row it is linked to.

Using a query like “SELECT * FROM Assembly WHERE Serial=‘abc’'” returns a row with an empty list for the PCB field, even if there are rows linked:
“PCB”: [ ]

This doesn’t happen if convert_keys is false.
It doesn’t happen if the column to display is a text field

Is this by design / a known limitation?