slide numbering in keynote is doubled

Hello,

every time I add in Keynote a new slide with the slide number, it contains the number two times (e.g. 33 instead of 3, 1212 instead of 12) in the same text box.

If a manually delete one of the numbers and then duplicate the slide, the numbering works fine (e.g. if I duplicate slide 3 after correcting from 33 to 3, the added slide correctly shows number 4). But it's annoying that I can't just add a "blank" slide by using the add a new slide option.

This issue happens with a slide template we use in our company. If I create a presentation with whatever Keynote native template everything works fine. So, I tried to recreate a new slide in one of the basic Keynote template, importing the graphic elements and the formatting of my company's template, and then I added this "new" slide template to one of my presentation. Result: if I add a slide with such new template the numbering works fine. However, if I apply this new template to the other existing slides (the ones that use the old template), their numbering doesn't work anymore (it appears on all such slides the number "01" whatever I do).

Does anyone have an idea on why on the earth this is happening? I tried many approaches without success. Apparently it has something to do with slide number placeholder characteristics, but there is no way to change this that I know.

Thank you for any help you would be able to provide,

Dario