Bugs, or unfinished coding, when players take long rests or level up?

Last night I told my table to level up. This did not reset their HP or class features. Why wouldn't a level up automatically trigger a reset of HP and features? Because then I went through each sheet and tried using a Long Rest to reset everything, which fixed HP but not class features; I had to manually reset everything else. Is this a really big bug in DDB or coding that's never been addressed?

It's neither a bug nor "unfinished coding"; per the actual rules of D&D, gaining a level does grant any healing or reset any features that require resting to reset. You need to actually take a rest to do that.

The concept that leveling up resets all those things is a computer game trope unrelated to D&D.

Looking at the PHB, https://www.dndbeyond.com/sources/dnd/phb-2024/creating-a-character#LevelAdvancement, doesn't actually clarify WotC's official perspective. There's nothing about character levels in the DMG. If it's "expected" that a rest is required to reset features and HP, especially in the case of a character gaining a level, I think that should be explicitly stated. My assumption doesn't come from a computer game trope because I've been playing for more than 40 years but only recently started using DDB, and the automation isn't working as I expected. That's why I'm asking.

You litterally linked to the level up page and NOTHING on that page specified that you regain all your abilities on a level up. 

Abilities and spells and how they reset are laid out in the rules across classes, and other sections of the books. 

Spell Slots. The Bard Features table shows how many spell slots you have to cast your level 1+ spells. You regain all expended slots when you finish a Long Rest.

 
Pulled from the Bard class, it says spells reset on a Long rest and that is it. Not a long rest and a level up.

Again not a coding error.

Why do you think a level up automatically heals & resets all abilities?

What video game/other TTRPG/DM gave you this idea?

You are free to house rule a level-up as a long rest, but it isn't an actual rule in Dungeons & Dragons 5.5e.