Master flow header is empty, cannot edit

I want to change the formatting of the flow header for all parts, but when I open the default flow heading master it is empty and I cannot draw any frames (I select the text frame button, the cursor changes to a crosshair, but when I drag nothing happens and the cursor reverts to normal). The preview in the panel shows a green frame, but when I double click it gives me the empty layout. Interestingly, flow headers appear in the part, but if I create a new default master they disappear and I can’t edit that master either.

Additionally, in the part master pages I can modify the existing frames but I cannot draw new text frames.

Dorico 2.2 Windows 10

Welcome to the forum, nsprangers. I’m sorry you’re having problems with flow headings. If you start a new project, do you find that the default flow heading looks as you expect there? If so, please zip up and attach the project you’ve been working on, and we’ll take a look and see if we can figure out what’s going wrong.

I created custom master files to work around this issue and now I can’t recreate it: new layouts with default master pages behave normally; setting existing layouts to default masters also doesn’t reproduce the issue. I will include files in the future.