Why can I sometimes perform multiple undos and other times only one on DJI Modify?

DJI Modify supports multiple undos for low-quality models, with a default maximum of 30 undos that adjust according to available computer memory. 

For high-quality models, if no mesh adjustment has been made the rules for undos are the same as for low-quality models. However if any mesh adjustments have been made the default undos is one.