After setting the font to 10pt you save the signature and then after refreshing the page and changing the signature again you will see that the blank space will be 12pt

If you set the signature in Preferences and set the font to 10 pt, there is still 12 pt in the empty space (empty side for future text)

For example, you can check this problem as follows:
After setting the font to 10pt you save the signature and then after refreshing the page and changing the signature again you will see that the blank space will be 12pt
Or try setting the signature to 10 pt, refresh the page and write a letter. You will see that the blank space in the signature will be 12pt

As per checking with Verse L3, it is the default behavior of the 3rd party editor (TinyMCE) that is used in Verse. If you do not add any text, the font change will not be saved. You will notice that if you change the font and then click somewhere else and then click back to the same spot, the font is reset.

A workaround could be for the user to add an empty space and change the font used for the space. That will cause the font for that line to be saved.