It is replace targetting logic of combat computers?
aggressive has "low_health_bonus = 5000" and defensive has "low_health_bonus = 50"

Range finder has same "low_health_bonus = 50" for all.

Hmm... I think that may have been a tweak that slipped in there from another thing I was trying out. I will fix this today.

I have a fleet of Corvettes and Destroyers, the Destroyers are set at longer range and they do separate. However the bug (?) that the longer range ships just sit there not shooting is still present. It eventually got to the point where my Corvettes got chewed up and my Destroyers were just sitting there.
I think this is caused by the fact that the Destroyer closest to the Corvettes was in proper range and could fire. Unfortunately, the other Destroyers were forced (by fleet formation behavior) to sit so far behind the first Destroyer that they were out of weapon range. So I guess this is a problem with formation and fleet behavior.

What if you try setting them for a lower range? I specifically used the range 5 as a base with 10 unit increments, because a lot of weapons are ranged starting from 10 with 10 increments. This means that generally you want to have a range setup of 5 units lower than the shortest range weapon you have on the ship.
Please send me the save gamethat displays the behavior you're talking about. I'll take a look.

Sorry for not answering earlier. I don't have a savegame. I already made sure that my weapon range is greater than the rangefinder.

As you can read in your comments and in the Paradox forum and on reddit, this is a common error. It might even be in the base game and most often happens when attacking stations. I guess that's because the station is, well, stationary and doesn't move into range itself. Enemy ships however might move closer so before you notice the error it gets "fixed" by movement by one of the fleets. The BB guy is also dealing with the issue and has posted a forum thread about this in his mod forum. If I understand correctly there is no 100% fix for this yet.

This bug annoyed me a lot so i did some testing with corvette/destroyers with torpedos
vanilla:
Ships with longer range weapons will stay a little behind but ultimatly enter the melee ball regardless.
Eventually all ships will engage stations it just takes a while for some.

With RF all ships on 45:
Choosing a different range didn't change anything about the following behavior.
After entering combat ships slow down to a crawl. With topedos this zone happens to often be just inside the range of some L torpedos making those ships engage normally while the rest of the fleet lags far behind until they get into weapon range.

Edit:
changing these parameters seems to make it work better
preferred_attack_range = 5/15/25/35/45
return_to_formation_distance = 10/20/30/40/510
attack_move_pattern = orbit/charge (for 25+)

I am using a mod which adds some new ships into game.

"New ship classes & more"

Range finder Modules are available for original ships in game. Corvette, cruiser etc

Range finder module doesn't appear on those added ship classes.

Carrier, Dreadnought etc

Not sure if this is something you can change?

Cheers!