I have to remount the SD to be able to write to it, after a while it becomes read only.

So. I have a 200GB sandisk card (3months old) and if I try to write to it
(1st whith Solid Explorer) If its a small file (like a picture) its fine...But when i try a bigger file (10+MB) only 2-3MB will be written to the sd (and this applies when trying multiple files, each will be 2-3MB of the 10+...
(2nd with ES file explorer) The whole file will be written to the sd, intact, but It is very slow (10-15MB/s, but the card is rated to be able to write 50+MB/s)
I have given sd permission to both.

I have root
If I unmount the card then mount it agin, apps wont be able to read it (music player can't find music on sd, google photos the same)

But for a few minutes I am able to write to the sd with solid explorer at full speed. Then it resets agin...

Very strange.
Phone: Vernee Apollo Lite

Android: 7.1.2.  madOS 1.1