The release notes of 1.14.3 Pre-Release 2 lists this: "Reverted enchanting system to how it was before 1.14"

This change no longer allows multiple types of protection enchantments to be applied on the same armor piece. This behavior was marked as Working As Intended on MC-141961, and has been in the game for 1.14, 1.14.1, and 1.14.2. Is this an unintended effect of the change in 1.14.3 Pre-Release 2 or is it now intended to longer be possible (despite being intended in previous 1.14 releases)?

Changing something rather major like this in a bug fix (pre-)release is very haphazard.