Hmm, that result would also match what you originally posted – like the ‘sand.png’ is not there.
Is there something wonky with the actual ‘sand.png’ file? But Vegas somehow has it cached for preview? Go out to explorer and see if the sand.png is viewable with something else. Recreate ‘sand.png’, maybe. I’d try to ‘replace’ that image in Project Media.
Or put your mask back into ‘SunnyBeach’ and try ‘sand.png’ BELOW it.