A little while ago, I came across δε for the first time on Duolingo. I managed to find an explanation (it may have been this or another one, I think it has been asked and answered more than once):
the final ν is retained before vowels and the following consonants/combinations: π, κ, τ, ψ, ξ, μπ, ντ, γκ, τσ, τζ.
(https://forum.duolingo.com/comment/48250654/), but I still had no idea what makes these consonants being treated differently from the rest, so I was happy to find more explanation about it:
https://forum.wordreference.com/threads ... ν.2664524/
They call those that would drop the ν "continuant consonants", and I think I can understand the difference now.