On IBM keyboards, scan code sets 1 and 2 have a long make code and lack a break code for the Pause/Break key. Moreover, when Ctrl is pressed, the make code is different, although break code still doesn't exist.*
In scan code set 3 there does appear the break code, but this set isn't used much.
What was the reason for Pause/Break to be treated so specially to not even have a break code?
*See detailed tables in e.g. this spec for EM83053DH keyboard encoder. I've checked experimentally that description for Pause/Break is correct at least with set 2 active.