Mapped fields aren't pre populated on Quick Create Form

[jonckhje]

Dear reader,

Today I'm working on a pretty straight forward challenge. I have this subgrid on my custom entity and want to enable adding new records using a Quick Create Card.
So far so good.

But, there are two fields that need to be pre populated on the QCC, but whatever I do, they remain empty. They're neatly defined in the Mappings section of the relationship:

Having seen this work for a million times, I can't come up with a reason why it shouldn't this time. 
Do you?

Thanks in advance,
Jeroen