When bulk merging versions either the Affects Versions field.

Recently we decided to make the version list more manageable by merging our nightly build versions into their subsequent released versions. I spent a long time today doing this, for 5 years worth of builds. I then decided to mark our released versions as released in Jira - until now we've been tracking releases externally but not in Jira. One of the release screens flagged an unresolved issue: PIRATES-47084. Then I noticed something bad: the Affects Version field has been cleared out, now reporting 'None'! The Fix Version changed from a deleted version to the corresponding released version as I expected.

I made a sampling of issues and found that for most issues, both Fix Version and Affects Version were cleared. (PIRATES-32690). In some issues both fields changed. (PIRATES-52709) In some issues Fix Version changed and Affects Version was cleared. (PIRATES-47084) In some issues Affects Version changed and Fix Version was cleared. (PIRATES-53104)
For some issues there is an entry in the change history, for others there is not. (PIRATES-32690)
Affects Version for PIRATES-52647 changed from 1.18.59.0 to 1.18.60.0 but in the same merge transaction Affects Version for PIRATES-52631 was cleared from 1.18.55.0 to None.

It appears that if the merge from and merge into versions were adjacent in the version list the field changed and if they were non-adjacent the field was cleared.