I know this is very late but I found this as the first thread on the subject after trying some of the things suggested, also repeated in the XS Official UnleashX Forum FAQS recently.
I thought I'd fixed a new UX logo by using an icon file size much smaller than the default ie. <10KB and it did stick for much longer but at some point it too was over-written. But now I think I have solved the problem with an alternative solution I've found accidentally if you do want to change the UnleashX splash logo rather than just switch it off.