Ok, I could reproduce the issue, under Windoze only (this OS is
brain dead !)... It occurs whenever you copy (CTRL C) an emoji and then attempt to paste it (CTRL V) in an input field...
Probably something to do with the clipboard encoding and the IME (input method) encoding...
Yet, the Cool VL Viewer offers you a workaround; it emulates the "primary selection" of Linux, including under Windows, so you can copy/paste
without using the Windows clipboard between viewer text and input line editors: simply select the text (or emoji) to copy with the mouse left button and drag to select what you want to be copied, then "paste" the selected text in the input text/line editor of your choice by pointing the mouse cursor into it and clicking the middle mouse button. The text will then be properly pasted and the emojis preserved...
Not sure I will bother to "fix" this Windoze idiosyncrasy, so better getting used to the Linux way of copy/pasting...
EDIT: a search on the web for
windows clipboard encoding issue returns many results...