Apple - SD card slot is always read-only

In my case, a physical misalignment is causing the read only status. Gently pushing the disk away from the display reliably makes my cards write again.

Slide the read/write lock to the middle neutral position and retry. I have the same problem sometimes...

The problem spontaneously disappeared for me. I'm leaving this as an answer since it might reassure some people to know that for some random reasons, at some point, things start working again. However, I was not able to pin down a specific cause for it, so I'm not going to accept the answer, in case that at some point someone comes with a rational explanation.