Automatic field updates not working explains very well the difference between fixed and automatic.
Is it possible to update a fixed field?
So far I have used the workaround: Convert to automatic. Convert to fixed.
But is there a ‘update fixed field’ that I am not aware of?
I also do “Convert to automatic. Convert to fixed”, but maybe there is a better solution.
See Inserting a Fixed or Variable Date Field - LibreOffice Help - fixed fields are meant to be fixed, that is they will not be updated. If you want to automatically update a field in a document, you shouldn’t make it fixed.
If I start of with a template-like document, which I want to edit and save as a new file, I might want to keep the date fields fixed in general, but update them once while creating the new file.
… just right-click on it in your file manager, and choose New instead of Open, which would treat it as a template for a new document - and that would update all “fixed” fields properly, as templates should behave.