RNA migrating slower than DNA on Formaldehyde Gel?
The inclusion of formaldehyde in the gel and buffer is to keep the RNA denatured (ie after heating the sample to melt the double-stranded stem-loops, just prior to loading the gel), in the hope that the RNA will migrate through the gel with an Rf proportional to its molecular weight (approximated here by its length). Therefore the correct size markers would also be made of RNA (and prepared the same way, by heating).
It is difficult to predict, a priori, how a double-stranded DNA fragment would migrate under similar conditions, without also including a lane of ds DNA size markers.
In other words I would only use DNA markers for DNA and RNA markers for RNA. By repeating this experiment (1 lane of DNA MW vs 1 lane of RNA MW) at different voltage gradients and different concentrations of agarose, you might be able to plot two standard curves that would let you predict where your DNA fragment is expected to migrate relative to the RNA transcript, but without that data I would not assume that a larger DNA fragment would run slower than its RNA transcript.
Even if the dsDNA was exactly the same length as the ssRNA (and pretending that the MW of the dNTPs and NTPs are identical), the DNA's MW would be twice as much as the RNA, but the negative charge on the DNA would also be twice as much.