Hello,
not sure whether it's a bug or a feature. I couldn't find any note on the changelogs when searching "aim" or "melee".
When hitting aim (right mouse button) during a melee animation the player will lock to aiming as soon as the melee animation finishes (can move freely but any weapon is being held for aiming without the player doing anything else). Hitting aim again or melee breaks the aim lock but this behavior is new and not shown anywhere in the option etc.
It feels like a proper aim lock feature (hitting right mouse button once like in other games with toggles) but it's implementation does not feel like it belong here.
AFAIK it started happening with update 33 on any mode, map, weapon, warframe etc.
PS. No ne of the just 8 tags fits my perceived bug's behavior. I myself would tag it with animation or controls. :-(