EDIT: I am sorry! This is not just for Long sword. This is for all martial weapons! I just checked the drop down list again. This means the error is that it doesn't register the Martial Weapon proficiency class feature from Hexblade (However as screenshot below shows, the Medium armor/shield registers correctly)
Step by step to recreate error of long sword not showing as proficient. (Did it twice to confirm)
Open and fully reset v2.28 zenith BETA3 RC example character sheet and custom tab.
Enter stats (8, 13, 15, 10, 12, 14) and set XP to 900 (Lvl 3)
Enter level 1: Sorcerer
Enter level 2: Sorcerer
Enter level 3: Warlock
Enter race: Half-drow elf (Set bonus stat to dex/con)
Set subclass sorcerer: Divine soul
set subclass warlock: Hexblade
Set worn armor and shield to half-plate & shield
In field 11. Attacks - under Attack option 1, select 'Long sword'
For me, this then shows up as martial melee, not proficient (Also says in properties, not proficient, versatile)
That's how I get the error. However, could this have something to do with the spelling of the name of the attack? 'Long sword' rather than 'Longsword'?
(To be clear, I do not type out the name, I select it from the drop-down list in the sheet)