I think it's age-related. I'm 66 and while I've been playing for 54 years now, about 10 years ago I noticed the same effect - playing about 45 cents flat. I had the issue with the Sterling Virtuoso and with both the Adams E1 and E2. Oddly enough, I didn't have that problem with my 967, but that was 20 years ago. Tried all kinds of things without spending a bazillion bucks on mouthpieces, so my fix was -- and it wasn't a cheap fix -- was to get Adams to build me an E2 about 50 cents sharper than they normally do. They did so at the factory to the point that you can't get the first valve out of its casing unless you take the valve button off. Just a little bit shorter all the way around.
It worked great. I've had my E2 for more than 5 years and I no longer have that problem.
U.S. Army, Retired (built mid-1950s)
Adams E2 Euph (built 2017)
Boosey & Co. Imperial Euph (built 1941)
Edwards B454 Bass Trombone (built 2012)
Boosey & Hawkes Imperial Eb tuba (built 1958)
Kanstul 33-T lBBb tuba (built 2010)