That's great! But a new one on me.
I do wonder if that Fritz box is accurately reporting the interleaving status, as what has happened ties in with what is normally expected from an interleaved (downstream) line and with the history of your line.
Thinks - up comes a dim memory of some modem/router or other that does report the upstream value of interleaving in both fields.
I say this because the expected minimum downstream interleaving depth on G.INP is 4, with the most frequent value being 8. It never completely removes it, and I've never seen anyone report it being activated direct from Fast Path.
Note that a few people rightly corrected me on my answer to your second question. Nobody on my answer to your first .