Hi Chris
Here is a youtube video that explains one way of doing the processing in PI:
https://www.youtube.com/watch?v=H2r3ZYqCaP8
This is after you have separately processed each tile.
Also he says use linear images.
I have done it with non linear and it works just fine.
I remember you use SGP?
In the framing & mosaic wizard, the default overlap is 10% don't go lower.