There was a recent update on the Pinephone which is related to storage. Probably this will fix the issue on the Pinetab as well so try to switch to devel and see. I am not sure though how in sync the developments for the Pinephone and PInetab though.
Also, I think the PInetab only has 32GB 🙂
EDIT: Sorry, I was thinking of the convergence edition of the PInephone 😅
I switched my PineTab to the development channel and it sees the correct size now. Thanks !