Kindle preview is a little quirky, plus there is an actual conversion process from ePub to the proprietary kindle format (not ePub). I'd recommend loading it onto the kindle itself for checking.
MY guess...based on old information... is that the kindle previewer may be hanging up on the position:fixed. IIRC the older kindles didn't support that but now do. Kindle previewer may have not been updated???
If you see the same problem on an actual kindle then you can try using this instead:
Code:
div.v-ctr {height:100vh; width:100vw; display:table}
If that doesn't fix it, then I have officially exhausted my knowledge of Kindle quirks. Maybe someone else knows more?