Blowguns on the dndbeyond character sheet automatically add DEX modifier as a bonus to damage. It should not, per the explicit example given in the 2024 PHB:

Damage Rolls
Each weapon, spell, and damaging monster ability specifies the damage it deals. You roll the damage dice, add any modifiers, and deal the damage to your target. If there’s a penalty to the damage, it’s possible to deal 0 damage but not negative damage.

When attacking with a weapon, you add your ability modifier—the same modifier used for the attack roll—to the damage roll. A spell tells you which dice to roll for damage and whether to add any modifiers. Unless a rule says otherwise, you don’t add your ability modifier to a fixed damage amount that doesn’t use a roll, such as the damage of a Blowgun. See chapter 6 for weapons’ damage dice and chapter 7 for spells’ damage dice.

I searched this forum and found this issue being brought up under 2014 rules, with someone deciding that NOT adding the DEX bonus was a "bug" previously. But I believe that having DEX modifier added by default is clearly NOT correct according to the rules. Furthermore, it complicates creating homebrew, and confuses the rules around True Strike.

If the Blowgun attack DOES HAVE an ability score modifier added to the attack (which is contrary to the PHB), then TS would have you swap that for your spellcasting ability modifier.

For Homebrewing, if I make a Blowgun that would use INT/WIS/CHA for attack, I both can NOT remove the DEX modifier, and can not add another one.

Is bug, yes?