Ah, well, this is an interesting one! Generally speaking, once you unlock the HP view reward from a particular Monster Manual entry, you should be able to see the HP of any monster that belongs to that race (or sub-race), whether or not you can actually kill that monster. The same, interestingly, also applies to active NPCs of that race - that is, any NPC coded with a CR. (Some of these NPCs can move around and attack things, but others are simply coded using the Monster AI because the devs need them to behave in a particular way that isn't included in the standard list of NPC behaviours.)
Taking the Chronoscope as an example, therefore, on Heroic and Epic difficulties, it is possible to see the HP counts of all five of the Abishai Henchmen and the Conjoined Devastator, as well as Nelle's Warforged bodyguard Retcon (who is typed as an active NPC largely, I believe, because he needs to be under the effect of an Otto's Dance spell while he's in the Phoenix Tavern) and the Warforged, Elven and Drow Stormreach Adventurers scattered around the tavern and the Marketplace (I'm not sure why these are coded as active NPCs - maybe because they need to be able to switch between "knocked out" and "awake" states?)
That's how things are
supposed to work. In practice, it doesn't always go like that. A lot of monsters which can't have MM entries, and a huge number of valid active NPCs, don't have the extra bit of code added to their data files which activates the Monster Manual HP view. I don't have an exact list of these, largely because it's so difficult to check.
You
should be able to see the HP of the five abishai in Legendary Chronoscope, but they haven't been given the correct code. It's worth noting that you should also be able to see the HP counts of Nelle Gann, the Stormreach City Guards, and the Human and Dwarven Stormreach Adventurers, but those are also missing (this applies to all three difficulties, not just Legendary.) The Conjoined Abishai Devastator, meanwhile, is missing from the MM entirely on Legendary difficulty, which explains why
@WielderofGigantus can't see its HP count.
Here's a quick overall summary, then.
- If a monster has an MM entry, you'll be able to see its HP (assuming you've unlocked it for that monster.)
- If a monster doesn't have an MM entry but should, you won't be able to see its HP count. (If you're checking, this is a handy indicator that the monster is missing its entry.)
- Active NPCs and monsters ineligible for an MM entry should have a visible HP count, but that's no guarantee that they actually will.