I recently bought at 100 year-old Cape-style house. The floors were
sagging and the culprit appeared to be the center beam, which was being
supported by 3 6x6 posts that had rotted from the bottom up over the
years. The worst sagging was probably about 2".

I've jacked up the beam and re-supported it with adjustable jack-posts
and all seems well except that the joists don't seem to have moved with
the beam. In other words, the beam is far more level and the floor
directly above the beam is just as level, but the floors simply have a
"speed bump" shape in them where the beam is supporting them but they
quickly sag off to either side of the beam.

The joists appear to be in good condition, though they're certainly
original and they are supported both by being mortised into the beam
and additionally by bulky (1/4" thick?) 90-degree angle braces. I'm
trying to figure out why the beam moved but the joists apparently did
not.

N.B. - I did jack the beam up relatively quickly. Over the course of
maybe a month. Could that have something to do with it?
Help appreciated, thanks.

JPC