On Reddit A7III and A7rIII owners reported a major bug. The Sandisk 128GB card is not working when you use them on Slot Number 1. IThe problem only appears with the 128GB card version and only with Sandisk.
Hendrixj92 pointed out:
I contacted Sony. After a quick explanation of the issue, the rep found other reports of this happening. It appears to be a firmware issue with 128gb cards. They are working on it.
I am sure Sony will soon fix this via FW update.
Source from sonyalpharumors