Text field aligned in Master page not aligned anymore when changing content

[vronizor]

Hi,

I have a master page with a text field which is aligned with the top-left corner of my margins. When I apply the master to a page and change the content of the field, the text box moved depending on the characters that are typed into the box. How can I prevent the text box to move, i.e. how can I "anchor" one of its vertices to a given X-Y coordinates? I have tried "alignment" but that just sent the text box against the edge of the document. I've tried "transform origin" but no success either. Finally, I've tried to remove any optical adjustment for specific characters, but to no effect. Below is a video of my issue.

Thanks

Affinity Publisher 2.5.6
MacOS Sequoia 15.1.1
MBP M1-Pro, 14-inch, 2021

[MikeTO]

Cross linking two threads.