Good morning Abigail.
Looking up the word "behaviour", I find that it relates to "actions" and "reactions". Things that have happened. Actions that have taken place. Past tense.
Saying that behaviour is undefined, makes no sense. Until something has happened it isn't behaviour.
If you predict that a certain thing will (future tense) display a certain behaviour, and you get it wrong. It didn't display the "wrong behaviour", or "no behaviour". The action the the thing displayed (past tense) was the behaviour. You were simply wrong in your prediction.
Therefore, you can't say a "behaviour is undefined", because an action or reaction doesn't become behaviour until it has happened, and when it has happened, it can be measured, and is therefore not "undefined".
It may not be predictable--though in this case, I (rashly) bet the outcome has been the same since Perl 1; maybe you would confirm that?--which kinda makes it a bit predictable.
It may also be unexplainable. Which may explain why the explainations are so lacking.
It is, however, OBSERVABLE! Huge red font omitted.
If a phenomena can be observed, it can be questioned. The strongest defining impulse of the human being over our non-sentient co-inhabitants, is the ability and need to ask the question WHY?.
You may well be correct in reaching your conclusion that asking why is not "worth your effort" or "likely to result in a you reaching a satisfactory conclusion", but that is your conclusion. It satisfies your criteria.
Is it your assertion that because you have reached a conclusion that satisfies you, that noone else in the entre world is allowed to think about this, or discuss it any place where you might observe that discussion?