Urgot R cancels on opponent invincibility

As Urgot whenever I have hit an opponent with R before they are below 25% , they use stopwatch (unsure if this is stays for zhonyas, taric ult, and other invincibilities but i think i have experienced it then as well) and they go below the 25% threshold, R does not activate to kill while the count down remains on them. This has occurred every time a stopwatch has been used after the first cast of R.

edit: maybe i should add that when the invincibility ends and urgot r is still "on" the second cast fails to go through. Sorry that wasnt clear