Just to report I have had this happen last night, in 1.1.2 with kOS installed but no RO. The parts I noticed it happening to were parts with KIS inventory space, and in one occasion I opened the KIS inventory and saw the mass readout mysteriously 'filling up' while editing in the VAB. I've not been able to reproduce it, I chalked it up to random error and moved on, until I just now read this here. So perhaps it's KIS, or an interaction between kOS and KIS?