I'm aware these are not solutions (such a strange phenomenon!) but mere workarounds:
- Instead of saving as picture you can copy your group and paste it as picture on the slide; font stays.
- Instead of inserting a text box you can type your number into the shape. If you save that shape, the font stays.

Still, I'm intrigued with this issue and hope you get a 'real' answer!

Hi


I also came across this weird behavior (bug?). In my case, Calibri (only when selected as body font in the Design tab) got changed to Arial automatically when exporting a grouped text box to png. Export was successful when another font was selected as body font (tested with Arial) and the text was written in Calibri.



I found the following to work, which is somewhat an extension of judithvg's second suggestion:
Instead of a textbox, draw a rectangle. Go to Format Shape > Shape Options > Size & Properties > Text Box and select the radio button "Resize shape to fit text" as well as deactivate the checkbox "Wrap text in shape". This will pretty much emulate a regular textbox.


Grouping and exporting as png worked for me without the font changing any more.


Still not a solution though...