Unfortunately, the “default text” option of a field is extremely limited — it only gets filled in if you are manually creatIng a record in that table while you’re actively working in that table as your frontmost tab.
This is a huge bummer because, as you’ve figured out, if you create a new record through a linked record field, your default text WON’T get entered. Not the behavior that somebody would expect to happen.
In fact, the default text option of a field is so limited that it doesn’t even get filled in if you create a record through Airtable’s API.
So the “default text” option can’t really be depended on, so I would recommend ignoring it and pretending like it’s not there at all — unless you know with 100% certainty that you will ONLY be manually creating records in that table while you are actively working in that table as your frontmost tab.
However, the great news is that you can use Airtable’s brand new automation feature to create default values in your fields upon creatIon of a new record, and the new automation feature ALWAYS WORKS, no matter HOW you create your new records: via the API, via linked records, anything you’d like!
So the automation feature CAN be depended on, and it’s the way to tackle default field values in Airtable.