OK here’s what worked best for me. Instead of the above Airtable formula field, try this:
"" & {HTML (from Spotify Embeds)}
This causes Airtable to decide that this field must be a string type.
You know, now that I think I really understand this situation, I think it’s a bug in Softr that it can’t figure out that this value is supposed to be a string. I’ll ask.