Yes, this is pretty much what I've seen -- it's simply using more battery on standby with no obvious culprit. And like you, my battery life remained excellent after Oreo and only fell off after the latest update.
Part of me suspects it's due to a Spectre patch, but of course I have zero evidence for it. My own problem with doing an FDR is that I have the darn thing setup exactly as I want it -- all of the extraneous HTC Sense stuff turned off, my smartwatch configured perfectly (which is more of a pain than it should be), and the Alexa stuff optimized for me.
So again, just grrr. Not the end of the world, it's just I'm really busy this week and it's a bad time to both have poor battery life and need to do an FDR and get everything setup again. Another week and it wouldn't bug me so much. Toss in the fact that I'm not 100% certain the FDR will actually resolve the problem.
I was having poor battery issues after the 8.0 update. I was getting great battery on 7.1.1. I was hoping that see you the 7th. They will fix my battery issue. I had done a factory data reset after upgrade to Oreo. So I'm going to do a factory data reset tonight and hope that fixes it if it doesn't then there's something seriously wrong with the bass Oreo for u11 that needs to be fixed