weird behaviour of the E string with a 4001

[vaccastracca]

Hello everyone.
Something weird happened to my 1975 4001 bass in these days.
First i have to point out I'm not a fan of changing strings every 3 months.
In fact, my Rickie has the same strings set since 9 years! (a Rotosound Roto Bass 40-100)
After 9 years, it's like playing with flatwounds, and i just love the tone!
But in these days I wanted to try a DR set, which I never tried before.
I just changed the E string, and I didn't like it one bit, so I put back my old E and decided it was not time for a strings set swap!
To my surprise, now the old E sounded different!!!
More high frequencies, and less warmth!
The punch was there, but I lost the "magic" sound I had before!!!
This thing puzzled me to say the least!
I mean, I know of people that take old sets off, washes them, or even boils them in water, and then when they put them back on, the strings sound almost like new for a while!
But I simply took the string off for maybe half an hour and put it back on without doing anything to it!
How can it sound so different than before????
Any opinions welcome!