I used the Arduino IDE with a modified font : Cascadia Code. I have modified preferences.txt to include the line:
editor.font=Cascadia Code,plain,16

All was well, until I installed the preview build of Windows Terminal from the Windows Store, and i believe a modified version of the Cascadia Code font was then installed as well. When I started up Arduino IDE after this, some function names were garbled and unrecognizable. I confirmed the following:

1. this is a result of setting 'Cascadia Code' editor font in Arduino IDE. When I set another font, the issue is not present.
2. The issue is purely visual. When I copied the source with the garbled charactes to i.e. Visual Studio Code, the source code was intact, what is should be.
3. This is in Arduino IDE 1.8.12. I tested and confirmed that the issue is also present in the latest "hourly build" of Arduino IDE, at this time version 1.8.14 dated 2020年7月3日 04:33 which is on the website labeled "LAST UPDATE 23 July 2020 16:25:17 GMT". However the function names setup and loop were not garbled, but instead the module name Serial was.

Again I suspect that it has something to to with installation of Windows Terminal as I happen to know that a newer font "Cascadia Code" was installed along with it. This is Windows Terminal version: 1.1.2021.0. The release notes says: This version of Windows Terminal ships with Cascadia Code 2007.15.

Screen shots:

Arduino_1 8 12

Arduino_1 8 14_Hourly_Build_2020_07_23_04_33