I think Pehkui or Campanion is causing this issue. Pehkui changes your hand reach when you change size, and Campanion increases your hand reach with spears.
https://www.curseforge.com/minecraft/mc-mods/pehkui
https://www.curseforge.com/minecraft/mc-mods/campanion

The spear works even when the player is scaled, although it seems that the extended spear reach does not grow with the player size.

Campanion also has longer hand reach for entities, so you can borrow the code from there if needed.

The reach code has been removed