To clarify, lets assume that ending 3 is 8 bars long, as is intro 3.
Native behaviour of the Genos is to finish playing an ending in it's entirety before moving to the next selected button (intro, ABCD, or ending).
But, for an intro, it will move to the new setting on beat one of the next bar, regardless of how many bars are left in the intro.
So for example, you press ending 1 (or anything else) right after starting ending 3, it will play all 8 bars of ending 3 before switching. BUT, if you play intro 3 and then immediately hit something else, it will switch on beat one of bar 2.
I understand it's an annoyance, but I'm afraid I don't know any way to change it. At least, without editing the endings themselves. That's above my expertise level.
Adam