Our problem
The backlog in the next-gen software shows "Your board has too many issues", instead of showing the normal backlog screen, while there are only 1000+ unclosed issues, after a change in the workflow.
About our project
The project contains 7000+ tickets.
Among them, 5000+ are closed (Done) and 1000+ are unclosed.
Before the change in the workflow
We had four status in the workflow (Three standard ones + one customized):
To Do, In Progress, Resolved, Done
Where Resolved is an in progress status.

After the change in the workflow
We now have five status in the workflow
To Do, In Progress, Resolved, Ready for testing, Done
Where Resolved and Ready for testing are all in progress status.
Our question
Is this a bug, or is there any problem with the backlog when adding additional status?