I also had simmilar issues with Sandisk cards, especially wafter they were worn some.
In my contact with Sandisk they referred to their speed test, but it would only write 1 GB (on a 64 GB card) and they would not accept a test that wrote more. I did not want to spend more time on it, except that I concluded that their speed statements are false, when they do not test the whole card.
For the same reason I switched to Samsung too.