Cant unmerge some issues that had been previously merged

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

There seems to be issues that cannot be unmerged in issues/{groupId}/merged view. All I get is "already being merged". Only appears to happen when issues appear inside issues{groupId}/merged view that are not associated with the same groupId therefore a grouphash record cannot be found.

### Expected Result

Should be able to unmerge all issues.

### Actual Result

Cant unmerge all issues. Some show as "already being merged".

### Product Area

Unknown

### Link

_No response_

### DSN

_No response_

### Version

_No response_

Start by tracing the unmerge action from the issues/{groupId}/merged view and inspect how it handles issues whose grouphash record is not found. Reproduce the "already being merged" response with an affected merged issue, then verify that every issue in the view can be unmerged successfully.

Written by the indexing model from the issue text.