Testing all (US) keyboard accessible characters, my testing suggests only colon is affected.
From comments above, chrome browsers on other platforms are not affected
I just installed Chrome Edge browser to test, and can confirm it is similarly affected (colons not visible in Discourse, but visible on other web pages)::.
In Chrome now, and typing a bunch of (for me invisible) colons produces ::::::::::::
And it does look like overriding fonts in Devtools font-family temporarily fixes the problem, causing the above colons to appear (this is achieved only by deleting specifically the ‘Helvetica’ font reference, with no issues leaving either or both Arial’ and ‘Sans-serif’ in place )
Possibly for people who have Helvetica installed as a system font, this breaks something specific to the Discourse-in-Chrome context, but not for other browsers?
EDIT Confirming that uninstalling Helvetica as system font does resolve the problem in Chrome, and Chrome Edge browsers.
Not really a viable workaround, as like many people, I do use Helvetica in CAD and other graphical applications, and my formatting in existing documents will be messed up if another font was substituted.
Searching Google it seems both windows and Mac users are elsewhere reporting similar though not identical problems with Helvetica font in relation to Chrome on some websites.