Rick Dipper wrote:

The joists should nto shrink as they dry, cause they should have been
kiln dried before installing.

What I think is that the packer put under the joist to hold them level
has either come out, or crushed. I would put a straight edge on the
ceiling below, to see if its no longer straight.

The ceiling below is still 100% level with no signs of movement.

The other reply suggests that joist shrinkage *is* normal, though I must
admit I am surprised by the extent (6-7mm over 25-30cm) of it.

--
Tony