Unfortunately, Airtable designed their forms with that limitation. You should definitely send an email to support@airtable.com to request that they change this behavior.
In the meantime, there are only 2 known workarounds for this issue:
1. Make your primary field (in the linked table) a formula field that combines all the fields that you would ever want to see on the form.
2. Use the external form app Fillout, which natively communicates with Airtable in real time, and gives tons of advanced features that are all Airtable-specific. This is one of those features that Fillout provides.