Have you checked that the typed character that’s missing isn’t e.g copied/pasted from somewhere? It may be an ‘odd’ international character that isn’t available in that system font, everywhere.
Does the font support the character that’s missing? Presumably yes (sometimes), because it sometimes works - but if it’s a system font that you’ve not set up any kind of import process for, then characters may behave differently on different systems…
A different font may fare better, but best solution is to ensure that the font you want is e.g a web font (run through font squirrel or equivalent) that you’ve imported via @font in CSS