I assume that InDesign will see the element on the left master still as part of the right one (or the other way around).
What can you do?
Just position the master element a bit away from the spine.
0.001 mm could be sufficient.
Thanks Uwe, yes, I already did that and it works, but I was wondering why it happens?
Thanks for the response anyway, much appreciated.
Why? Rounding errors.
And if one element is touching the opposite page then also this element will be reflected to the page where the master is applied.