Numbers document with text in Arial - blank cells. Pages document with Arial - visible. Mail.app new message with Arial - blank text. All reliably consistent.
... and I haven't found a work-around, or many other people with the problem.
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Same symptoms per thread in MS Outlook.
But then new email in mail.app, change the text to Arial and it vanishes -- still selectable, but not visible. Regardless of dark-mode, size of side bars, screen resolution etc. Completely reproducable.