In FireFox, advanced form contents are empty when saving it as an empty string

[ghost]

Bug description
The form textarea appears empty, even when it should be populated by the advanced form.

Steps for reproducing
Steps to reproduce the behavior:

1. Open module settings with Firefox
2. Switch to advanced form.
3. Delete contents of textarea element.
4. Save
5. Refresh (F5)

Expected behavior
Textarea is filled with advanced form.

Actual behavior
Textarea is empty.

Screenshots
In inspect element, the textarea HTML seems to be filled but yet appears empty.

WordPress version
5.4

Module version
2.3

Additional context
This doesn't happen in Google Chrome but happens on FireFox 85.0.1-1
Editing and saving the HTML will make textarea contents visible again.
Ctrl+Shift+R fixes the problem but disabling cache in developer tools does not affect it.