The pressure capsule is not sealed, and the bellow leaks air(at a slow rate). The rate of air leakage is proportional to the rate of climb or descent. The bellows are mechanically connected to the indicating needle. Delays (is it sticky in general ?) might occur due to the instrument being worn out.
But abrubt altitude changes WILL cause reverse sensing.
My guess is, you've got a VSI that needs replacement!
The bellows don't like debris, especially if you fly doors off in dusty areas. Amazing what you can collect !!
Then again, it might be groundeffect. Try it out at altitude. If it behaves the same way as close to the ground, replace the VSI and buy all your students beer :-)