yeah not unexpected really, I had the same experience.
110mbit attainable when I first unlocked modem (within 2 weeks of service live)
Then within a week of unlocking it, it dropped to circa 90mbit in one go probably an install.
Then not long after the attainable was down to the low 70s.
After that point further drops happened but were much smaller, and the long term average on that pair was a sync in the high 60s, until it dropped to below 50mbit at which point openreach treated it as faulty and I am back in the low 70s on my new pair since.
Those people who have managed to keep 80+ after crosstalk many I noticed had initial speeds above 130mbit attainable, so there is a quite a drop between no crosstalk and populated crosstalk.