You are probably correct on that. I have added warm soapy water to the tumbler and put a vacuum on it and back on the tumbler and it foamed out to the point their was no more liquid movement. Open it back up and went back to a watery consistency.
If I were to vac seal a bag of soapy water and put it in the tumbler though It might not have enough volume to expand to cause a foaming issue.
Good point though.