Changing to a custom font doesn't work reliably

### Windows Terminal version

1.21.2701.0

### Windows build number

10.0.22631.4317

### Other Software

_No response_

### Steps to reproduce

* Install custom font like "Fira Mono" or "IBM Plex Mono"
* Try to configure it in Windows Terminal

### Expected Behavior

I expect the font to be changed

### Actual Behavior

Windows Temrinal claims in cannot find the font when pressing the save button. This also happens if I change the font directly in the JSON, as soon as the config file is saved Terminal says it cannot find the font.

Although, after I trigger something (I did not find out yet what it is, maybe it's just randomly, it suddenly works until i restart terminal again)

Reproduce the issue with Windows Terminal 1.21.2701.0 on Windows 10.0.22631.4317 using a custom font such as Fira Mono or IBM Plex Mono. Trace the font validation during settings save and JSON reload, then verify that the configured font is accepted consistently after saving and restarting Terminal.

Written by the indexing model from the issue text.