Using a spawn egg on a mob without a baby variant spawns an adult mob of the same type without a weapon

The bug

When using a spawn egg on an existing mob that does not have a baby variant, a new adult mob is spawned, but without a weapon.

This happens to every mob that spawns with a weapon by default:

• Skeletons
• Strays
• Pillagers
• Vindicators
• Wither Skeletons
• Vexes

In 1.15.2, using a spawn egg on these mobs would not do anything at all.