NHF – new heroes for Heroes V 3.1.

Respawning – which is now a familiar feature in Heroes 5 – takes place in the tavern. If implemented, it would be simpler to return 30% of the hero's army when they are bought back from the tavern, provided the same player who previously controlled the hero is the one who buys them back. Otherwise, the more workarounds you add, the more inconsistent everything becomes.

In my opinion, there's no point in doing it this way. This doesn't make sense for hero recruitment with an army. No one will ever have more than 14 units, because wasting the entire army is always a bad idea (and returning 30% won't fix that), and the intermediate values between 14 units and a full army won't be that great in terms of the amount of damage inflicted on the enemy.
An automatic respawn without a tavern and with an initial non-zero army, which doesn't depend on the army the hero had before being defeated, seems more effective in this regard. Like Nicholas in the fifth campaign (but not with a thousand skeletons, hehe). But yes, this function probably needs to be disabled for neutral creatures; otherwise, you could easily gather a large army out of nowhere, which would be an effective replacement for necromancy