There are many features that do not work when using software rotation (the hardware does not support rotation). We strongly recommend against using rotation as it will operate properly in only a very limited fashion.
The driver is doing a software rotation by providing a framebuffer that we claim is the rotated geometry to the system, then RE-RENDER that framebuffere to the actual displayed framebuffer that is then shifted out to the display. This re-render step consumes a LOT of graphics bandwidth and does not function with a lot of other display features. Quite frankly you are lucky that as much works as does with the rotate feature. I am surprised you are only having issues with composites time feature. If you want that to work you will need to run in the non-rotated mode.
Sorry the answer is not better but the driver is doing the best it can given the limitations.