Probably the most confusing thing in Falcon, that one can setup things which are not functional, but it all boils down to the same thing related to the Falcon hierarchy – re-triggering envelopes only happen on keygroup level. If you link a dummy keygroup to the ADSR on layer level and then assign that ADSR to the delay wet parameter you have a monophonic envelope to control the delay, of course the envelope has to be inverted so that when the synth stops playing the delay will become audible, probably a multi-envelope would be a better solution as the ADSR stops working after the release phase (and there is no infinite release time in the ADSR or DAHDSR).