As a glider pilot, this doesn’t surprise me at all…We are always on the lookout for those clouds and learn to interpret when they “pull” and when the cloud is dissipating (which means there is little chance of updraft under it).

Also, not all cumulus come from heated floor, it could be formed by different in temperatures and pressures in upper layers that pull hotter, stationary air from below, but there is no clear updraft from the ground up to be expected (usually common post-warm fronts or in troughs).