I don't think ddDLM is lowering your connection rate, its some sort of noise lowering your actual and attainable speeds while synced.
ddDLM would reset the connection and either raise your SNR profile or raise the Interleave depth (not visible on a Fritzbox)
Either would mean even after a resync your connection rate would remain lower.
ddDLM can also adjust PSD and such.
if you look at the line itself, Noise isn't changing but bitloading is.