If its a 1099-MISC I'd be wary, to say the least. Without the details its hard to say but...
Since 2020 (year?) the 1099-NEC has generally been used for non-employee compensation (hence the NEC) as a way to distinguish earned income from non-earned income (1099-MISC). Earned income requires that payroll tax be paid (FICA and medicare). 1099-NEC is considered earned income. Solo 401k contributions, like all 401k contributions, have to be made with earned income.
1099-MISC includes income such as prize money, collected rent, etc. Couple links below on 1099-NEC vs 1099-MISC- I'm sure you will find other articles on line if you search.
https://www.mysolo401k.net/form-1099-ne ... ributions/
https://www.businessnewsdaily.com/payro ... s-1099-nec
I'd say setting up a solo 401k without having an active business is probably running afoul of the rules. And I'd say that making contributions to a solo 401k with 1099-MISC income that is not derived from business income (business associated with the solo 401k) is running afoul of the rules - especially if that business doesn't report any income.
When I report contributions to my solo 401k on my tax filings the business income (1099-NEC) goes on schedule 1, line 3. The deductible part of the self-employment tax goes on schedule 1, line 15. And the solo 401k contributions goes on schedule 1, line 16. Note that entering an amount on line 15 requires I attach a schedule SE, which I do. Also have to fill out a schedule C.
If you don't do all the above, entries on schedule 1, line 16 (and line 3) it may trigger a closer look at your submitted tax filing.
Unless you are confident in what you are doing and don't mind an audit....
Well, I wouldn't do what you are doing - it wouldn't be worth the potential hassle to me. To me you may have dodged a bullet by removal of that money. My two cents.
P.S. And don't forget, even if you have more than one 401k going - the max contribution in a year to all of them in aggregate in 2022 is only $20,500 so if you contributed to any 401k (or 403b) in 2022 you would be over the limit -assuming your plan is legit, which I highly doubt.