Here's my 2 cents.
I fly the A320, and the 1G value is calibrated, not matching actual local acceleration. Typically, the force of gravity would be measured less than 1G near the equator, and I don't know the value one would read sitting at the poles, since I did not go there. In subtropical latitudes (East and South East Asia), the g displayed on the FMGC is less than 1, EVEN ON THE GROUND. Could it be due to slingshot effect as we get closer to the equator?
Typical values in flight always showed me less than 1G, so assuming that the Airbus books are correct and precise, disconnecting the AP would indeed revert to maintaining 1G instead of ALT, thus causing a very slight but noticeable climb.
If this does not happen on 330/340, maybe this minor flaw was corrected by software, but I don't have any experience on those types.
Should not be much of an issue anyways, how long do you want to have the aircraft flying hand off without an autopilot?
Flex