Patch notes 4-6 weeks ago claimed the trash respawn problem was fixed, but it isn't.
Fortunately, there appears to be a 100% reliable work-around. The problem seems to happen when you wipe in one instance of Denova (e.g. Denova 1) and get put into a different instance (e.g. Denova 2) when you re-enter. The workaround is to leave one person (dead or alive) inside the instance until someone releases and re-enters. That ensures that you stay in the same instance you were in (with dead trash).
This doesn't always work when depopulating instances. You can get an error message that you can't enter an instance even with live people inside.