Well you have not named the chemical so I don't know why I should bother.
Valuejet flight 592.
But it does not have to be a fireball accident. spillage of even common chemicals can cause damage to airframe and systems causing issuews later. At a lower level a diversion costs money delays passengers and increases risk due to non-normal operation.
Just so. Even water is a potential problem.
In the 757 cockpit after a flight landed my I and my daughter were chatting to the pilot (he even let her sit in his chair
). He mentioned that he had once had to delay a flight because water was leaking from a package containing live elvers. Elvers are a particularly edge case: if fresh water then no problem, if salt water then he would have grounded the aircraft for inspection. (it was fresh water)
And, of course, mercury has "interesting" and subtle effects on aluminium alloys.
EDIT:
Note that a serious incdent or accident normally requires a chain of events ot actions. A oxidizer alone may not cause a crash but if something else is wrong it may be a vital part of the chain. Banning such items is a important means if refucing the risk.
Google James Reason swiss cheese model
You made an extra hole in the cheese.
Precisely.
Any arguments to the effect that "I ran into a road without looking and wasn't knocked down" should be ignored. There's a good argument the person making such arguments should be ignored too.