Investigate the SpriteFont class in MonoGame.Framework/Graphics/SpriteFont.cs. Focus on how font styles (regular, bold, italic) are handled during font loading from .spritefont files. Look at the content pipeline's SpriteFontProcessor and the FontStyle enum usage. Check existing comments in issue #4149 for any hints. Consider testing different font style settings to isolate the bug.
SpriteFont Styles Not Working · MonoGame/MonoGame#4149 | Good First Issue