Does sound like that but could also be bleeding off at an injector as well. You can depressurize the rail and pull the rail with injectors and then turn key to KOEO position and can check to see if the rail pressurizes again and watch the injector tips for any fuel leakage. Should have no leaks. If the injector o-rings at the rail also do not leak then it is like the pump or the fuel pressure regulator leaking off. For flex fuel rig:
https://www.rockauto.com/en/moreinfo.php?pk=438163&cc=1424247&jsn=924
Did you already do a fuel pressure test and then shut off key and watch to see if the pressure holds for at least a few minutes and does not bled off quickly?
We recommend that you put all your rig(s) info in your signature so we all don’t have to guess at year, make, model, engine size, trans, rear axle locker…….etc. While you may have posted pertinent info in your first post, you may forget to do it again and the info is needed by us anyway. Makes it way easier on all members that attempt to help, to have that info right up front. It is also recommended that you take a pic of your RPO codes, on the inside cover of the glove box, and post that image in your signature. All this info will greatly help those that will try to help with a problem.
If there is any other info, like upgrades, mods or recent mechanical/maintenance work done, that would be helpful in any post as well. Too many times a question is asked by the poster and after about 10 responses we find out that the poster has done recent work to attempt to fix the problem or work that was done that SEEMS not related to the problem.