Just ran into this strange behavior on macOS with the
Poster word in the font family name suffix like "Fontname Poster". Any other two-words family name works ok but not a Poster as a second word. Looks like the text engine treats this word as a style (but not a family) name?
For clear test I exported the same set of outlines twice under the different family names: "Testm Poster" and "Testm Display". This is how one of the exported fonts looks like:
Family name: Testm Poster
Weight class: Regular 400
Style name: Regular
Full name: Testm Poster Regular
PostScript: TestmPoster-Regular
Style group: Testm Poster
Adobe font menu show me the wrong styles order for Testm Poster family:

And TextEdit doesn't show Regular and Italic at all:

But if I install only pack of four base fonts (RIBBI), than TextEdit show all of them just in a wrong order:
Is this a known feature/bug of the text engine and should I be concerned?
Comments
https://typedrawers.com/discussion/comment/55711
Did you check that each weight has a unique usWeightClass value (e.g.: Light: 300, Regular: 400, Medium: 500 etc.)?
Thank you, I will try it.
NameID 20 (CID findfont name) is certainly obscure. NameIDs 21 & 22 are weight-width-slope compatible family and subfamily, respectively. In many cases they will correspond to 16 and 17, unless the family has styles that differ on variables other than wt-wd-sl.
With the second option, do you mean figuring out which fields are triggering macOS, and changing just those? Yes, that might work technically, but creating inconsistencies… feels wrong to me, at least.
I think #1 is the option I would pick, but… arguably all these options suck, right?
By name ID fields – I mean to change all that needed to correctly display fonts on both macOS and Windows. But agree, I don't think I will go this way, even if it work.
At the moment I think about the first option and the question is how to name it. The problem is that when we add a new name, the users lose the clarity of the classification.
There are one more option, even more terrible:
Would you test these two families (IBM Plex Sans with different names)?
https://workupload.com/file/KvVErVyUGwV
I named them Testm WWS0 Poster and Testm WWS1 Poster.
I'm not a Mac user, so I can't test myself. But you can see the difference in Windows.
If the fsSelection bit 8 is cleared (WWS0), Text styles are not listed in their place between Regular and Medium. With WWS bit set to 1, they are correctly listed. Maybe that bit has some effect on macOS too.
Here the results from macOS 10.13.6
Adobe.
Both family's styles are in the right order, and it's better than my own previous test. I compared your and my own files but can't see any difference on a base name ID fields. Interesting..
TextEdit.
Regular and Italic didn't appear at all, like in my own previous test.
It is worth mentioning that TextEdit always displays non-RIBBI families in the wrong order (I checked it on system font families like New York and San Francisco), perhaps this is some kind of feature of it. But I expect at least to display all the family styles, even in a wrong order.
Another try: I unlinked the Regular and Bold styles, adding nameIDs 16 and 17 also where apparently not needed. See attached file.
Just checked, looks like unlinking didn't help.
TestX Poster showed the same result as Testm WWS1 Poster, in all applications.
https://optimo.ch/typefaces/next
The last files work for me (TestX Poster).
Both in Fontbook and Textedit "Regular" and "Italic" are listed on top, the others are sorted by weight.
Yes, Font Book on my Mac is also works well with the Poster subfamily.
Looks like your friend has a newer version of macOS than mine. I'm glad that the new version of TextEdit shows all font family styles, that's really good news!