I've got the same issue
I have a brand new very nice Nokia 6.1 and moved my SD-card from my old phone to the new phone.
But I can only use it as portable storage. I cannot use it as Internal Storage (so that it expands the Phones storage).
Could it have something to do that it is a slow performing SD-card?
I am using a ADATA 32GB SDHC 1 Speed Class 10.
ok, so why did nokia remove such great feature? i hope it get fixed