Text regions don't preserve their font unless a font is specifically selected for them. Let me explain.

If you open SMath, the font is what was selected at last session. For example we say it Arial.
You start to type some text into a text region without selecting font.
Because you like what you are seeing on the page you don't need to select a font.
Text is displayed in Arial font, it seems nothing is wrong here but that is not true.
Now save the file worksheet1.sm and close it.

Open another worksheet and change the font. For example select Courier New this time.
No need to save the new worksheet but ensure you have changed the font. Close the new one.

Reopen the worksheet1.sm, all your not-font-selected text regions shall appear in different font.
Because font of text region is not specified in the first place, they will appear in Courier New.
Checked the sm file with notepad, text regions don't get their "font-family" style automatically.
The font style is only added if you select the font from combobox even if it is the font already selected.

Didn't try on Windows but at least this is how it is in Linux.

Regards

PS: Don't know it it is intended and design choice. Maybe it has to behave like this I don't know.

This is an issue. Will fix.
Thank you!