Legacy query in task processor

I see there are two different implementation in workflow processing[/temporal-ruby/spec/unit/lib/temporal/workflow/task_processor_spec.rb]
1) context 'when deprecated task query is present' do
2) context 'when deprecated task query is not present' do

What does deprecated/legacy query mean here. For some reason, some of my workflows goes into an infitnite loop of
a.Processing Workflow task
b.Workflow Query task completed
c. Workflow task processed
d.Processing Workflow task
and goes one

But some workflows are successfully processed, where they follow a different lifecyle
a. Processing Workflow task
b. Workflow task completed
c. Workflow task processed

Can someone explain when a workflow goes into legacy task vs normal task?