Comments and suggestions share one id space, and a reply finds its thread by id. When two threads share an id, replies go to the first one, and only the first keeps the id.
Incorrect
The link expires after 15 minutesIs 15 minutes enough?Keeping 15., and a code after 10 minutesShould the code last as long?.
The link expires after :comment[15 minutes]{#ttl note="Is 15 minutes enough?"}, and a code after :comment[10 minutes]{#ttl note="Should the code last as long?"}.
:comment{reply="ttl" note="Keeping 15."}Correct
The link expires after 15 minutesIs 15 minutes enough?Keeping 15., and a code after 10 minutesShould the code last as long?.
The link expires after :comment[15 minutes]{#ttl note="Is 15 minutes enough?"}, and a code after :comment[10 minutes]{#code note="Should the code last as long?"}.
:comment{reply="ttl" note="Keeping 15."}