This issue has existed for at least two years now - effects from mythics and other sets that provide an effect "while in combat" are breaking in Battlegrounds (as well as certain environments like IC where it's common to die and respawn without ever leaving combat).
Items that definitely still have this bug include Death Dealer's Fete, Roksa the Warped, and the new Monomyth Ring. These items will sometimes stop granting their effects if you die in a BG and respawn without ever leaving combat.
@ZOS_Kevin I've definitely made a report about this before, but this issue is continuing to happen. This isn't a niche scenario either - its very common to die and respawn in PvP without ever leaving combat, and the items that this bug affects are all staples of PvP.