It’s a bit silly while it’s technically correct that expanding munitions were banned by the 1899 Hague convention modern hollow points could probably be fine, especially the fragmenting ones.

The reason these aren’t widely adopted isn’t because of any convention as these hardly stop anyone during war time, but because they aren’t as useful as you think for military purposes.

Armor piercing incendiary and high explosive incendiary rounds are used and I rather get show with a hollow point than an HE round.

The Soviets during WW2 used HE rounds as sniper munitions.