HI !

Might also depend of rig model.
But what you say is true. I have ICOM 7300.

Escape should stop CW that is ok.
Pressing a F key and then another F key should send both texts. F1 = OH1KH F2=599 should send OH1KH 599

But there is no feedback from rig and so we can not know is the CW buffer empty or not. HamLib's buffer can usually hold larger message than can be sent to rig buffer. How ever both buffers can have overflow.
So if you press F key too long and cause key repeat function to start it will cause buffer overflow and CW transmit may send "what ever...."

Maybe this could be fixed somehow by locking last pressed F-key so that it can not be repeated more than once.
Then release lock by a press of some other F-key or after timeout that is calculated from last F-key text and CW-speed.
That, how ever, prevents you placing 2 or more same F-key contents to buffer queue. Like as above example pressing keys F1,F2,F2 to get OH1KH 599 599 queued.

I have to think about this. I have noticed the same (just during HA dx contest now) it completely mess up the keyer.