Stories being moved automatically from inactive sprint to backlog

[M T]

I have exactly the same problem as described by here back in 2015. We are running Jira v7.2.2 and here is how the problem unravelled (I am pretty sure by following below steps the problem can be recreated too but cannot confirm that since have not tried):

1.   2 Stories were taken out of the active Sprint 1, re-estimated and added to Sprint 2 instead (which will be starting in 2 days time). The same developer name has stayed assigned to the stories.

2. The Sprint field in both tasks shows the correct value of "Sprint 2"

3. Once Sprint scrum board is refreshed the two items disappear from Sprint 2 (which as mentioned has not started as yet) and are moved to backlog. The two items are still showing Sprint 2 value in the Sprint field if stories viewed. At the same time the effort estimation total value for Sprint 2 goes down by the effort estimation total specified for the two stories.

4. I tried all sort of things, deleting/adding labels, components and different things but none of it work except for cloning 2 stories (including the links) which fixed the problem and the clones stay in the Sprint 2. I deleted the two original story tasks.

This has happened 2 more times and I was wondering what the potential problem may be here and whether any of you managed to get of the bottom of this issue. Creating clones and deleting original is far from sustainable solution. The thread from 2015 suggest re-indexing and I will speak with our Jira admins.

Many thanks in advance for any suggestion the forum may have.

M