Formula cell maodifies text

[KoenS]

If I place the cursor in a cell, then normally I can drag that cell up or down by clicking on and holding the little square at the bottom right and then moving it up or down, but sometimes when I move it up or down no other cells are selected. I just stay in the one cell.

Same when wanting to select multiple cells. I can only select one cell.

I genuinely don’t understand why this is happening. It is independent of the type of cells I’m working on. Cells that I can drag one day are no longer draggable the next. I’ve turned Sorting on and off.
I’ve tried refreshing the screen.

I’m sure I’m overlooking something very obvious… but what?