req.flash() returning different values on same scope

I set a flash value as:
req.flash('error', 'Invalid email or password');

but when i accessing this in appropriate section with console.log, the results are different:

console.log(req.flash('error')); //output: ['Invalid email or password']
console.log(req.flash('error')); // output: []

what is reason? why it is returning different result?

Start at the req.flash() entry point and reproduce the two consecutive reads shown in the report. Check the implementation and existing tests for how repeated reads of the same message are handled; done means explaining the observed outputs or identifying a reproducible behavior that needs a documented or code-level change.

Written by the indexing model from the issue text.