A tabulator (right-aligned) cannot be set at the right border of the text space with the mouse (it always vanishes in the moment you touch the right border with the mouse). The only possibility is to set it numerously in the paragraph settings
Now, that I tried to make this video, I recognized that it must have to do with the margins somehow. In one document it works and in another it doesn’t. Please see the videos.
Yes, I understand how tabs and margins work. But it is so, that you cannot set a right aligned tabulator at the right margin, when the margin is set to 0.8. You can accomplish it, when your margin is set to 1.5. You can see it in my videos.
My guess is that you cannot set the tabulators at any point, but only at certain positions, e.g every 0.25mm. So when the right margin is not set to one of these raster points (in my example it is set to 0.8 and not to 0.75 or 1.0) the tab just falls over the edge of the margin and vanishes into the abyss …
Now that I know this (and hopefully don’t forget it the next time), I could live with this, if it cannot be fixed. I’ll just have to fix the margins to multiples of 0.25 - and they would have been, if I had set them by mouse and not by numbers.