I can't see why such a warning would be put on cartridge boxes.
The use of .22 WRF in .22 WMR revovlers is a practice I've done many times. I used the magnum cylinder in my Ruger Super Single Six amd loaded .22 WRF for squirrel hunting. Why? The .22 WRF cartridges are slightly larger than .22 LR and easier to handle when my hands were cold.