Hi @PrashantIyer ,
Is the "Main Business Interest" field a LookUp type field in your Opportunities Entity, which reference values from the "Business Interest" Entity?
Could you please show more details about the Items formula within your "Main Business Interest" ComboBox?
Based on the issue that you mentioned, I have made a test on my side, and don't have the issue that you mentioned. Please make sure you have specify proper formula within the Items property of the "Main Business Interest" ComboBox:

Set the Items property of the "Main Business Interest" ComboBox to following:
Choices(Opportunities.'Main Business Interest')
then specify proper column name as display column value within this ComboBox:

Also please go to your Opportunities Entity, then check if the "Main Business Interest" LookUp field is bind to your custom Entity "Business Interest" properly:

Best regards,