Interesting thread. I thought I was alone or had gone senile and had forgotten something essential to queen introduction 101.
I have had queens disappear (5 x of last year's new queens) in April/May, virgins mate then disappear after laying their first patch of eggs, established queens vanish to be replaced by queen cells which hatch, the queen mates, then she also vanishes, and all hell break loose when uniting a small nuc to a colony using the tried & tested newspaper method with the queen ending up dead.
I have also had newly introduced queen cells torn down when this was the only QC in the hive - the same bees did this 4 times despite me introducing QCs from different batches/strains.
I've also had 3 x DLQs, I usually get 1 a year maximum, if at all - undoubtedly due to poor mating caused by the spring weather.
I have doubled the number of autumn queens and nucs I am raising, in the hope I will have more "spares" ready for spring than usual, but of course the other concern now all my hives and nucs have a viable laying queen (for now) is how well/often did they mate, and is their spermathica "full".
I have also collected nearly 30 swarms this year, and at least 5 of them were queenless. In two of these cases, I actually found the queen dead on the ground with no obvious injuries. (Both of these were on car park tarmac so were easy to spot).