Running a Level 10 Rogue who got his hands on a Sun Blade. I naturally have 20 Dex at this point and a proficiency bonus of 4, and it's a Finesse weapon.

My Attack Bonus should be +11 (5 Dex Bonus + 4 Proficiency Bonus + 2 from the weapon) but it came out as a +7. I figured out pretty fast what the issue was. The weapon counted entirely as a Longsword, despite the fact that in the item description says "If you are proficient with Longswords or Shortswords, you are proficient with the Sun Blade." Since my Rogue is NOT proficient with Longswords, only with Shortswords, the character sheet seems to think I'm swinging around a sword I'm not proficient in.

And yes, I am attuned to the sword.

It is outside the scope of the system to grant proficiency in a specific weapon - the proficiency system only works with general categories. Since the Sun Blade does not grant proficiency to general longswords, only to this specific Sun Blade, this feature of the weapon is not supported. 

You could customise the weapon on the sheet and add a bonus to the "to hit" box equal to your proficiency bonus but would need to remember to edit this whenever your proficiency bonus changes. Or, you could customise the Proficiencies section and add 'longsword' proficiency there - you just have to remember you're not actually proficient with longswords.

Thanks. I did the second, just have to remember not to swing any Longswords around. Should not be a problem...I have a Strength of 7, I'm not touching any melee weapons without Finesse.