It's as though the labels have lost their parametric link to the viewport scale or something. I created a viewport with a profile in it, with profile labels, grid text, etc. When I save and reopen the drawing, the labels are all tiny. But, if I copy the viewport to a different layout, they're fine again. Why would this be happening?
NM, from reading other posts on the board, I found that the layout print settings were set to 1:25.4..... So that was affecting the label display. I fixed it by setting to mm rather than inches and setting the scale to 1:1.