Describe the bug
This issue was first reported in helix-editor/helix#7129 including a reprocutions case. I did some digging there (helix-editor/helix#7129 (comment)) AltGr handling seems a mess on windows but I do think this may be a bug in crossterm.
Disclaimer that i don't have a windows machine or a keyboard that uses altgr at hand myself so I didn't test this myself but the original reproduction case was confirmed by multiple people in helix and we don't do any processing of crossterm key events.

I cameup with the reproduction case below but I can't test it myself:

To Reproduce
Steps to reproduce the behavior:

1. Open windows terminal
2. run the event-read exmaple
3. use a german keyboard layout and press altgr-7 to type {
4. Observe modifier Alt | Ctrl in the key event (with char {)

Expected behavior
A key event for { without modifier is emitted.

OS
Windows

Terminal/Console
Windows Terminal (stable and preview) and Git Bash (+winpty)