I think I figured this out. (this is a 2021 return)
Open the prior years return. Switch to Form mode and find the K1 for the missing partner.
At the top of the page you will see two check boxes: Final K-1 and Amended K1.
The Final K-1 was checked on my missing partners K1. I unchecked it. Saved the prior years return (2020 in this case) and started the 2021 return over.
Worked for me.
Hope this helps.
