[solved issue - see at the end of the description]

Previusly I used the next command through the BP animation_controllers, and the colored nametag displayed correctly in-game

/summon fur:zombie ~ ~ ~ minecraft:entity_spawned §aManolo

But now suddenly, if I create a new world with my custom pack applied, no entity correctly displays its nametag. I was already checking and everything is "correct"!

Empty nametag is shown in-game, which is displayed as a small transparent vertical line above the mob, as you can see at the attached image

SOLVED! I had to set the correct characters format. Sorry, it seems that at some point I changed it by mistake..