It is the attack speed bug! Whenever your hero switches map, their base attack speed(base attack speed+ agility bonus(white aglity only)) is reset! To test so, you can just go to bonus campaign, boost your agility for any hero to very high, then ungear him and go to another map, you will see his attck speed is reset to its original.
I remember when I was playing the bonus campaign 10 years ago, I spent days on bonus campaign to booster my Raxxar so that he can easily kill that ogre warcheif, but then i found out this bug and lost interest to the campaigns since then.
Now it has been 10 years, and the bug is still there, even afte “reforge”.
Seems you devs really don’t care about campaign players.