Generally speaking, bugfixes go into the stable branches. If the bugfix is particularly large, complex, or risky, it might be worth it to commit to master only. For this one, I think it's okay to commit it to the Plasma/5.12 branch too.

The preferred workflow here is to commit to the earliest branch and then merge; so that would mean committing to the Plasma/5.12 branch, and then merging the Plasma/5.12 into the Plasma/5.13 branch, and then merging the Plasma/5.13 branch into master.

However since you already committed it to master, that's not an option, and it will need to be cherry-picked to Plasma/5.12, and then Plasma/5.12 merged into Plasma/5.13, with no final merge to master.

But I'll let one of the more experienced plasma developers have the final say in this matter.