Using your example numbers, the February distribution that was deposited into the Roth IRA satisfied half of your RMD and the distribution in December more than satisfied the other half of your RMD.
Because the distribution from the traditional IRA in February was entirely RMD, this amount deposited into the Roth IRA constitutes an ordinary Roth IRA contribution despite you intending it to be a conversion. If you are eligible to treat the amount deposited to the Roth IRA in February as an ordinary Roth IRA contribution, you could just leave that money in the Roth. Otherwise it's an excess contribution that needs to be removed by an explicit return of contribution before the due date of your 2025 tax return, including extensions, otherwise it's subject to an excess contribution penalty.
Because it has been less than 60 days since the date of the December distribution, you can accomplish the the intended Roth conversion by indirectly converting that portion of the December distribution as you suggest. This will leave you with a net, non-converted distribution of an amount equal to your RMD.
The downside of having to do this is that the Roth IRA has probably increased in value since February, so the amount needed to be distributed to accomplish the return of contribution must include the net income attributable to the amount deposited in February. That attributable net income will be taxable on your 2025 tax return.
If you do the indirect conversion of a portion of the December distribution, you'll want to make a note in your tax records that the portion deposited into the Roth IRA in February was a failed conversion due to not completing the RMD first. Because the IRA custodian has no way of knowing if your RMD had been completed (by taking it from some other traditional IRA that you might have), they are going to report that portion on Form 5498 as a conversion. You want the note in your tax records in case the IRS ever questions the amount converted since the amount actually converted will be half of what is reported on the Form 5498 from the Roth IRA.
