Hi @madcar86 ,
Could you please share a bit more about your scenario?
Do you want to add new fields via clicking the "Click to add fields and forms" link in your Business Process Flow?
Based on the needs that you mentioned, I have made a test on my side, actually, it is actually a normal behavior with the "Click to add fields and forms" link in Immersive BPF. When you click the "Click to add fields and forms" link in your Immersive BPF, you would be navigated to a Classic Experience View.
Actually, when you create a immersive BPF on your side, it would insert your immersive BPF into Default Solution in your current Environment, and it would create a corresponding CDS Entity for this immersive BPF in your Default Solution:

If you want to add fields into the corresponding Entity related to your Immersive BPF, you could find the corresponding Immersive BPF Entity in your Default Solution as above, then you could edit this Entity, and add new fields into this Entity without using Classic Experience View.
In addition, you could also add new fields into the corresponding Entity through Classic Experience. You just need to switch to "Fields" option within the opening Classic Experience View, then click "New" button to add new fields:

Please check and see if the following blog would help in your scenario:
https://knowledgefrommanish.com/flow/business-process-flow-immersive-experience-in-power-automate/
Best regards,