Regression: Projects may be reverted to the default issue type scheme during the change due to race condition

Problem

When performing multiple issue type scheme changes at the same time, it can result in issue type schemes being reverted to use the Default Issue Type Scheme.

• This occurs when making simultaneous changes with multiple projects via from the Project Admin page: Project settings > Issue types > Actions > Use a different scheme

Steps to Reproduce

1. Generate 2000 projects.
2. Created a new issue type scheme, named 'test issue type scheme'.
3. Associated 1854 projects to the newly created issue type scheme 'test issue type scheme'.
4. Updated three projects simultaneously, originally associated with the default issue type scheme, and set it to use the new 'test issue type scheme'
   • Simultaneously via Project settings > Issue types > Actions > Use a different scheme.
   • In this case, I had 3 browser tabs open and quickly triggered the change at once using shortcuts to click "OK"
5. Review the number of total projects associated with 'test issue type scheme'.

Expected Behaviour

The number of projects associated with 'test issue type scheme' should now be 1857

Actual Behaviour

The number of projects associated with 'test issue type scheme' is 626. The missing projects were reverted back to use the Default Issue Type Scheme