- A reference guide and manual in Russian for Heroes 3.

and the same applies to Minotaurs (although theoretically their morale should be +2 (1 - from a single castle and 1 - their own)

it's not like that - they simply always have positive morale.
And - subjectively - the occurrence of morale drops, as well as abilities, depends on the balance of power (the so-called Murphy's Law).