I have to fight these kind of issues all the time. Unless your cabinets were spec'd as rustic, that's a reject. Replace it. No cost. Immediately. No question about it. It must be maple, eh ??

The wood is actually alder. Cabinet style was not specified as rustic, though I was made aware that it would have knots. However I do think that this type of imperfection should have been filled.

It doesn't need to be filled and don't accept that as a solution. They could have just selected another board and put that one in the rustic alder pile.

Reject it, you will forever be drawn to that natural imperfection.