In-plan Roth transfers

I have a client who received an in-plan Roth transfer election form from a participant on 12/27/21. The request was submitted to Nationwide the custodian on the same day.

All participant's contribution sources are record kept in the Nationwide account. No transfer of funds from outside of Nationwide. Nationwide shifted the funds from pretax source to roth on 1/3/22. Is there any guidance available that would allow the conversion to be taxed in 2021 as requested by the participant even though Nationwide will be issuing a Form 1099R for 2022?

I reviewed notices 2013-74 & 2010-84 which states taxed in year of receipt or year of conversion. Can one argue that the conversion occurred at the time of the election?

Can the Plan Administrator and Trustee adjust the custodian's records to account for payables/receivables among money sources?

Yes they can. In fact, the custodian agreed to change the fiscal year for custodial reports from 2022 to 2021. It's the 1099R they can't adjust.