I'm having a similar issue to one other people have mentioned where I have a box instead of numbers when I add dimensions to a drawing.
I have another document at a similar scale where the dimensions work just fine and I've confirmed all the settings are the same between the two files. It's not an issue with font sizing or general scaling and it's not draft mode. I've toggled draft mode back and forth and it hasn't changed anything.
I had the same issue that you described, but the dimensions did appear correctly in print preview. I closed the file and opened the file based on your post, and the dimensions now appear correctly. Now I have an issue with the same drawing, when I go to print preview and return to the drawing, the canvas is blank and it is displaying unnamed document. I think it's a wonderful program, and will continue to use it. I'm just starting with this tool, and the manual does say, "don't draw in layer_0" that maybe my problem.
When you close Print Preview the screen returns to "Unnamed document", which is with what LibreCAD opens, and still is not closed, but open in the background. The document you have worked on also still is open in the background. If you close "Unnamed document" you will see your work document minimized.
Best is to go to "Drawings" in the main menu tab and activate "Tab mode", then you get all open drawings and print previews presented.