I've made a set of four JPGs in which the camera was tilted 0, 90, 180 and 270 degrees. Their orientation is shown correctly as Top-Left, Left-Bottom, Bottom-Right and Right-top, respectively.
With TP9 Beta 3 Build 3952, the images are shown correctly in thumbnails, in the preview panel, in the Viewer as well as in Slideshows, both when 'Rotate based on XMP/EXIF orientation' is On and when it's Off.
The top row shows the images with rotation off, and the bottom row with rotation on:
(excuse the algae on the balcony railing :)
If anyone is still seeing this slideshow rotation problem, could it be that the images or their EXIF-rotation value are incorrect?
If anyone wants to experiment, the four images can be downloaded [link=https://www.dropbox.com/s/snpkmuf0fp1pqv5/Rotatetest.zip?dl=0]here[/link] (402 KB ZIP).