Net Unrealized Appreciation (NUA)

401k participant has appreciated company stock
Separated from service in 2021
He took (in 2021) a distribution of dividends (from employer stock)-  It's not clear whether he took the distributions while employer or after severing service. 

My understanding is (one of the many NUA requirements) is to make lump sump distribution in the a single year.  Does him taking a distribution (of dividends) nullify NUA?  If so, would he be required to wait under the next "triggering event" to NUA qualify?

All help is appreciated. 

Isn't a lump sum distribution defined as the balance of the person's account (paid in the same calendar year)? So if the entire balance is distributed in 2022, I don't see how a partial distribution in a prior year DQs it from being a lump sum.

Not exactly on point, but PLR 201241019 involved a similar situation with an ESOP participant who diversified in prior years, then took a lump sum upon termination, and the IRS treated it as a lump sum for NUA purposes.