I couldn’t find this discussed before, so it may be common knowledge. It always bugged me, how QuickLook & Finder on macOS would preview some fonts with numerals and others without. Couldn’t find any info about it other than “it’s just weird like that”. Now experimenting with vertical metrics I believe that if hhea Ascender + hhea Descender + yStrikeoutSize = not more than 120% of UPM, numerals will be shown. If the sum of these values is higher, they vanish. Could someone verify it on different fonts? Or if there’s a solid info on the subject, I’ll take that too, just to close this chapter 
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That is an interesting finding. I’ll check it out.
Huh, where did Rainer’s reply go? Anyway, I find it valuable to be able to influence the preview, some customers see that most of their fonts show numerals, and think those that don’t are broken. The previews are cached, but I alter the values in a .ttx file (always same file) and generate an otf, ttx adds a unique number, so the vertical shifts are visible.
After a few more tests I’m not sure it works the way I described. It definitely has to do with hhea calculations, and altering winAscent/winDescent I can see the numerals appear and disappear too. What makes no sense to me, is that if the sum of hhea asc and desc is the same, the outcome is different (1026 & -172: no numerals, 1023 & -175: numerals displayed, both sum up to 1198). It remains a mysterious issue 
Deleted my reply because it was too defeatist as to not raise your expectations too much, but Georg wants to try
But I am afraid it is not related to vertical metrics. I have had families with completely identical metrics, and they showed differently in QuickLook. It appears to be pretty random.