Hello,
I don't remember if the Sprint field in Jira Software always behaved like that or it changed in recent update, but now it contains all previous sprints, in which the issue was added, before being completed.
It looks like this on the Issue View screen
In the Agile Pane I can see the active sprint and completed sprints

I may be wrong but shouldn't the Sprint field contain only the current sprint, while the previous sprints where the issue was to be stored in a separate place?
Thanks in advance,
Pavel