With the "cache storage->on disk" option enabled, the animation preview miss all the frames that contains less to more or less 40x40 pixels. Try to make a new animation with a small resolution (i.e. for pixelart), for example 16x16 pixels: the preview doesn't work! Also, if you make a new project with a bigger resolution (i.e. 1000x1000) it still not work if you don't paint at least a 40x40 (more or less) area... frames with less of a certain number of pixels are reads as "empty" from the preview. This issue is only present with the "cache storage->on disk" option enabled, setting it to "In-memory" there are no problems and ALL the frames are displayed in the preview, also if there is only one pixel in the image! Sorry for my bad explanation!
Created attachment 114985 [details] Bug 397213 - quick test file CONFIRMED Application Version: Ubuntu 18.04 bionic (Linux) Details: Low document resolution does not play well with on-disk caching, will not properly play back animation. For the sake of future testing, a krita file is attached that quickly sets up the conditions of failure. (16x16 simple animated image)
*** This bug has been marked as a duplicate of bug 415367 ***