So I've looked around on this topic and it's still an issue, albeit a minor one. My M8 phone is unlocked/rooted, and S-OFF'd. I am currently using ViperOne M8, which is awesome!
When I boot to HBOOT after installing new firmware the OS line is filled in, but after booting to Recovery and going back to HBOOT, the OS line is now blank. I've been told this is a bug in TWRP, and not a bug or issue anywhere else. I've tried 2 different approaches to fixing it. One entailed flashing a new ZIP (or flashing something), but the instructions stated that I needed to boot to Recovery while in Stock Recovery, which my phone does not do. It's only when I use a TWRP Recovery that I am able to go into any Recovery Menu. The other one had to do with flashing a zipped firmware, but even though that flash worked just fine, it still didn't resolve the OS issue. I have tried looking all around, but either I'm missing something, or it's just not knowing enough about what I'm reading to recognize the right answer.
So please, will someone tell me HOW do I get rid of that pesky TWRP bug that causes the OS line in HBOOT to blank out after only 1 visit to recovery post-firmware update?
I would appreciate ANY info you can give me.