Shield is incompatible with spear because the spear charge attack have right click priority.
Ways to fix this will be give the priority to the shield, or/and making both jab and charge attack works with left click while there is a shield in the offhand. If the player is stopped or walking, left click will do jab attack, but if the player is running, moving on a mount, falling or flying with elytra, left click will do a charge attack.
Or just adding another key to active shield guarding, like crouching or auto guard when mount from Bedrock Edition.
I know that some of what I’ve said might be considered WorkAsIntended/InvalidFeatureRequest. In that case ignore that. The important fact is that in Bedrock Edition spear + shield works perfectly even on mounts, but in Java Edition the shield is just useless.
Comments 2
I’ll confirm this so Mojang can see, but this is very likely intended. If so, then a hoplite-like loadout is not realistic without modifying the components on the spear, but it would take no great effort to get that working (although an option to charge attack instead of jab attack with left-click would be nice).
Works as intended, mainhand right click functions take priority over offhand ones should both the offhand and mainhand items have right click functions.