Epic is aware of it; in fact, Epic rep Rykard posted a questionnaire attempting to nail down the exact issue. We're checking in with Epic for an ETA on the fix. Until then ... try not to play Horde mode in splitscreen.
Update: Epic Games tells us, "Getting this resolved is a priority for Epic. We are collecting information internally, sourcing as much as possible from the community and exploring potential fixes. We do not have any recommendations to share with players just yet."