Upgrading worlds from versions before 1.4 to any version after 1.4 (including the present version) will cause skeletons and zombie pigmen (converted to zombified piglins) to lose their weapons. Pre-1.4 worlds are old but any world since the introduction of the anvil file format has the ability to be directly upgraded to the present. The cause of this old bug is that there was no equipment NBT data before 1.4 and so it assumes that upgrades from these versions should have no weapons when they actually originally were wielding weapons. Below are the pictures before (1.3.1) and after (1.16.4 Release Candidate 1) an upgrade.