I'm an integrated/OEM type user so this might not apply due to your client-centric project approach, but I understand your dilemma. We also do projects for customers which creates similar issues.
Look backwards from ten years in the future, for your own answer.
I've used Inventor for years...
Sounds like you're using the wrong type of property (iProperty) in your title block.
Make sure you key from a MODEL Property and not a Drawing Property.
I'm using "Properties - Model" <PART NUMBER> in the title block and it works fine.
Important footnote: I've noticed it automatically keys...
Sorry, it cannot be done (yet), at least according to ADesk's official response when I last submitted this request/question about one year ago.
No way to "dynamically" pull this field into the drawing.
I found a VBA workaround posted YEARS ago for a previous version if INV by (or at) either...