I have a +1 longbow, and proficiency through the Bracers of Archery. My character’s DEX is +4, proficiency bonus is +3, so I would expect the attack bonus to be +8 (+1 for bow, +4 for DEX, +3 for proficiency). However, it is showing as +5 (DEX + bow). I tested with a regular short bow and it is showing correctly as +7. The longbow proficiency is correctly listed. I’m not sure what is causing the proficiency bonus to be dropped when applied to longbows, even though I have proficiency.

https://www.dndbeyond.com/profile/zacharymelnick/characters/57026827

That character is using a homebrewed knockoff or the bracers of archery. Whoever created it didn’t attach the necessary modifiers and will need to correct it for you. (Either that or you will need to equip the factory versions.)

I created the homebrew bracers of archery. I did so by cloning the original bracers of archery, and replacing the +2 damage bonus with +1, and then removing the attunment requirement. Everything else should have been cloned from the original bracers. The homebrew bracers of archery is adding longbow as a proficiency correctly. This proficiency isn't getting picked up, even though it shows up in the list correctly.

Quote from zacharymelnick >>

I created the homebrew bracers of archery. I did so by cloning the original bracers of archery, and replacing the +2 damage bonus with +1, and then removing the attunment requirement. Everything else should have been cloned from the original bracers. The homebrew bracers of archery is adding longbow as a proficiency correctly. This proficiency isn't getting picked up, even though it shows up in the list correctly.

 
Its because you removed the attunement requirement, causes a glitch. Remake it from scratch and it’ll work.