I took one of the "blessing of..." feats that is supposed to give +1 Con and advantage on Con saves. It gave me +1 Con, but did not apply the new constitution hitpoints bonus to levels 1-4 (+1 took me from 17 Con to 18 Con). I couldn't tell from the combat log if I got advantage on Con saves.
For reference, it was a Hill Dwarf Paladin of Tirmar with 17 Con (18 Con after taking the feat).
HP progression was as followed:
Lvl 1: 14
Lvl 2: 24
Lvl 3: 34
Lvl 4: 44
So basically, it started as 1d10+4: +3 from CON, +1 from hill dwarf. The first level is an auto "10" and then 6 (assigned in place of a d10) +4 per level after that.
However, after reaching 18 CON, I should retroactively get an additional +1 hp/level (1d10+5), which means my health should have been 48 at Lvl 4.
Here's the relevant 5e citation:
PHB 177:
If your Constitution modifier changes, your hit point maximum changes as well, as though you had the new modifier from 1st level.
From my little test with the leveling system in the game menu (without playing it),
another Feat called Enduring Body that also gives +1 Con. and +1 extra HP per level,
- did not give any HP for past levels when going from 13 to 14 Constitution ;
- and did not give an extra HP for that very level when I took that feat.
1) So there is the Constitution bug, as it should give you HP retroactively per level,
2) and the Feat currently is mediocre even if taken "early" at level 4 (which is the earliest possible). I know Tactical Adventures cannot legally reproduce the equivalent feat from 5e, but maybe feat Enduring Body should be changed to be better.
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