Version of the script
Seems to have always been the case.

What is the current behavior?
As documented @format_output = 1 "returns numbers formatted with commas". This hides its other behaviour, which is rounding decimals. Usually, we round to 0 decimal places.

If the current behavior is a bug, please provide the steps to reproduce.
Compare sp_QuickieStore @format_output = 1 and sp_QuickieStore @format_output = 0.

What is the expected behavior?
Either don't round or have documentation that admits that we do.

Which versions of SQL Server and which OS are affected by this issue? Did this work in previous versions of our procedures?
See above.

IMPORTANT: If you're going to contribute code, please read the contributing guide first.
https://github.com/erikdarlingdata/DarlingData/blob/main/CONTRIBUTING.md
Sure, but it's up to Erik to decide if this is an implementation issue or a documentation one.