Does preservation in ethanol alter leaf litter mass?
It's very likely that you should have at least minor changes in mass.


The reason is because ethanol can extract a number of substances from leaves (e.g. tannins, pigments, etc.). 


However, you could still easily double check on your own.

You could still figure out the mass of the CPOM even without knowing the answer to this answer directly.


Drain and collect the EtOH. 
Determine mass of separated CPOM.
Determine the excess mass present due to any leaching/dissolving from the CPOM by using density = mass / volume (or ρ=M/V) with your collected volume of ethanol. 


EtOH has a density of 0.789 g/mL  at 20°C. 70% EtOH has a density of 0.885 g/ml at 20°C. 
Determine the volume of the collected EtOH.
Determine the mass you'd expect of "pure" 70% EtOH given your volume (m = ρV )
Determine the mass of the collected EtOH
Subtract the expected mass from your calculated measured mass. The difference (if any) would represent mass leached/dissolved from the CPOM.