nonono this is working as intended. the game regeneration works on 2 second ticks just like rogue energy, mana reg as well. same with hp reg
if you got an item which has "restores 5 mana per 5 sec" you get 1 mana per sec, or 2 mana per 2 second tick. not 5 mana ONLY every 5 secs. The same is with warlock demon skin. if it say generates 10 health every 5 secs you gain 4 health every 2 second tick which translates to 10 hp/5 seconds. there is no extra ticks for 5 seconds in the game