Some weapons specifically say they will increase stats if used as a main weapon - if you look at the weapon abilities it will say that. So switching it to a sub weapon will lose that bonus.

sub gives (dont know exact value) like 10% of weapon ATK stat or something. and check apssives - some works always some works only as main weapon. thats it I think;) Generally pioritize all main weapons to lvlup, after that - lvl up subs

For instance, I try swapping a leveled weapon to sub slot and noticed a decrease in overall stats. Do sub weapons or elements have different formula?

Thanks in advance.

How do sub weapons' stat affect the character?