It's possible this is lightning-specific, but I suspect not. Moving an event that's scheduled in floating time by dragging it causes it to be put in the default timezone.
Presumably something similar happens for Zulu events. For the moment, we'd like everything created in Lightning to be pinned to the default timezone. However, any events in a file that have some other modality (floating, Zulu, pinned to another timezone) should preserve that modality when modified either by dialog or drag-n-drop.
0.1 is now targetted at no dataloss on self-generated data. Since we shouldn't be generating any floating events at the moment...
I'm pretty sure the patch on bug 323696 will solve this.
Fixed by bug 323696.