Restart the launcher.
As the question title suggests, there is a glitch in the launcher that is causing Unreal Engine version 4.16.1 to be read as a preview version (specifically preview 3) complete with the “This is a preview version, do not actively develop on this version” warning. I don’t know what could be causing the launcher to mistake a stable non-preview build of the engine as a preview version. I’ve tried reinstalling the engine version but the launcher still mistakes the stable non-preview build as a preview build.
I can confirm the same issue.
UPDATE: Exiting out completely from the launcher and reopening and logging in it seems to fix the issue but only temporarily. It quickly reverts back to reading version 4.16.1 as a preview version. Still have no clue what’s causing it.