I over contributed after tax 401k in 2021, then I withdraw excess money in 2022 Feb, there is some capital loss during the withdraw. How to report this loss to IRS?

return of excess contribution:

before tax due date including extensions: 

positive earnings allocable to the excess are taxable on the Line 4b for the year of the contribution. negative earnings are ignored; in which case, for purposes of basis, consider the original requested amount as returned.

positive earnings removed are penalized 10% if you are under age 59 1/2.

@zfyang