I'm using the default font (unchanged, no font set) in MonoGame with Myra.

Here you can see what it looks like after typing S, followed by no spaces, followed by one space, and followed by two spaces:

Here you can see the difference in spacing after typing S followed by one space, or followed by one space and any character:

I don't know if this is a kerning issue or related to custom layout of some kind, but you can repro this by typing a space at the end of a textbox. Ideally, the spacing should look the same with or without the 's' at the end.