I guess it’s rather due to the system fonts version, not particularly it being “open fonts” (they’re anyway on Android by their license).
What version are you on? the GSI is still at Android 10 - so it could be due to missing unicode characters for newly introduced emojis in versions higher and equal to Android 12L (see the german language link [2]).
- Can't receive emojis (could be the same)
- Emojis installieren [2]
I’m not a Viber user, it has App specific Emojis too… they should show if used. So if it’s not showing, a user just sent you an unicode char the system version can’t render, but not due to /e/ or Lineage changing the font, just AOSP didn’t have it either in their fonts version.