there're multiple alternatives and solutions for the situation you're asking, I'm going to explain my favorite one for each situation I can think off:
(1) The 2 branches are integrated in the "Release branch" AKA "/main" and you don't want to subtractive them.
In this situation you have to create a new task in Jira, link it to the task that has caused the issue and set it as a regression if it's the case.
Now that you have a new task in Jira you can create a new branch in Plastic. This new branch will base from the last cset/label in the "/main" branch.
Develop the fix and integrate it in the next release when it has all the QA on green.
(2) The tasks can't remain in the "Release branch" and they must be subtracted
You will perform a subtractive merge of the changesets created in the "/main" branch, so the "Release branch" will be reverted to an stable point.
The Jira task is reopened, even re-estimated.
The developer will continue the work in the task branch to adequate the code with the new requirements.
Once the task is complete the regular branch-per-task cycle goes on.
Hope it helps!