It turns out this is a two-part problem. First, there is a change to Windows 10, where when you install a downloaded font, it's not placed in C:\Windows\Fonts unless you install it for all users. This feature allows users who aren't Administrator to install fonts (which is a good thing for lots of people on Windows 10 machines that are locked down).
Inkscape does not yet take into account this new way of listing fonts, but until they change it, you have to install the font for all users.
Case 1: Re-install for all users (downloaded fonts)
If you installed a downloaded font with Install (as opposed to Install for all users), you'll find the font in C:\Users\<username>\AppData\Local\Microsoft\Windows\Fonts. Just right-click and select Install for all users (but you need to become Administrator for this to work):
Case 2: Microsoft Store fonts (not possible)
In my case, the Ink Journal font came from the Microsoft Store and it doesn't show in that local folder or even C:\Windows\Fonts as above. A supposed solution was discussed in Reddit and leads to a Microsoft forum, and is quite heavy:
If you do not see the font listed, then, in the Fonts folder, on the left hand side, click on the option “Download fonts for all languages”.
I tried it, and Windows 10 took a long time to download lots of Supplemental Fonts. However, in my case, I still don't see Ink Journal from the Microsoft Store in any of the directories where I can right-click and install it for all users.
To try to narrow down where the font file for "Ink Journal" is, I searched using PowerShell and couldn't find anything like a *.ttf in the C:\Windows tree.
In RegEdit I found lots of entries for InkJournal (no space) when I searched (not for novices). Eventually, I found an entry that was C:\Program Files\WindowsApps\Microsoft.136853439117B_1.0.1.0_neutral__8wekyb3d8bbwe\Inkjournal.ttf (surely a unique key on my machine).
TL;DR
If you search for your font (or any *.ttf) using Windows search inside C:\Program Files\WindowsApps\ you can probably find the Store-installed .ttf easily. Since it's installed as an App by the Store, it makes sense finally it would not be under C:\Windows.
Using that info, you can Install for all users as above.
Current status of Inkscape
The crux of the problem seems to be that Inkscape depends on fontconfig, which apparently doesn't read the Windows registry to get font info (an issue raised in 2015). A fix to Inkscape to include user-installed downloaded fonts in ~/AppData/Local/Microsoft/Windows/Fonts is on the way.
But I'm pretty sure it will NOT solve the Microsoft Store font problem, since those fonts don't wind up in that directory (nor any obvious directory). The request for fontconfig to support the font registry in Windows is over 4 years old, which doesn't look good for Inkscape supporting fonts from the Microsoft Store any time soon. At this point, it seems it's not possible to use them in Inkscape.
