If you're on ECI now you were before, no engineer involved when switching ISPs.
Interleaving is entirely down to DLM, the ISP has no control. Also as you've now identified as being on ECI with a 6dB SNRm, even if you are banded it would make zero difference as you're maxed out.
I suspect switching ISPs had nothing to do with this, something has changed to reduce the quality of your line and the increased error rate has caused Interleaving to be applied, reducing your speed. Losing 7-10Mbit from one neighbour suddenly causing crosstalk with your line is not at all unusual, sadly.