Private Use Area characters vanished in Lyx editor

[Márton Payer]

Dear Developers!

I'm a Windows user. Since the version of 2.1.1, Private Use Area
characters vanished in Lyx editor. I have tried the version 2.3.7
(Windows 10), these characters disappear on this interface as well.

And these characters not appear neither in the exported pdf, nor in
the pasted text from Lyx inserted to other editors.

Thank you for your attention, Marton Payer