From time to time I have this problem. I dont know why? Couldn't find any sollution around internet.  But By chance, I just enclosed the entire object I want to work on using the Laso Tool or Rectangular Tool and then when I tryed using warp, everything was ok. But only using these tools. If I don't "encircle" the object, the image, it will turn upside down again... ... View more

Windows 10 and having the same problem here , I've reinstalled Wacom drivers and updated graphics card driver etc   ... View more

You might want to check the default image interpolation setting in Preferences > General . You may find that changing in the options bar has overcome an issue with the default. Glad you have it sorted though Dave ... View more