I bit the bullet and bought a Huawei 128Gb Nano Memory Card today.
It didn't work at first, then I went back to look at the packaging and realised it MUST be in the right-hand slot, with the SIM-card in the left-hand slot.
The other way around does not work (but you won't see any errors or anything).
As others have said, this is a cracking way for Huawei to get money out of us... I hadn't read any reviews saying that the P30 was incompatible with regular memory cards, which cost a quarter of the price of Huawei's cards. Grrr...