Instead of changing bearings for every cut, would it not be much simpler to just adjust the depth of cut?
Maybe I'm too tired to understand the problem here. Unless you're using some extremely difficult to work wood, a bearing'ed (that a word? lol) rabbeting bit is foolproof. You have a quality router table. Use some scrap to help figure out the correct direction to feed the wood then have at it.
Or maybe your router bit is super dull?
It's possible! Realistically it was probably a combination of all factors. I no doubt glue starved the joint (I've got a nasty habit of doing this, even on long grain), the pine was sub-par for sure and very well could have had different moisture content, and if I would have made the splines larger the splines might have helped the joint withstand it.
I thought my splines were completely inadequate, that was obvious and that was the only error. Perhaps I should change the thread title to "...and why I think they failed."
What if.........The pine had high moisture content and shrank. With the outside corners fixed in place by the spline, the inside edge of the pine on each side of the joint would tend to pull apart. Could it over power the maple - I'm not sure but I think it did. It's not a joint problem per se.