When mending all four pieces of armor at once at an EXP farm or other way of gathering EXP, the following occurs;
EXP flows equally into all four pieces of gear until one piece is full.
The expected next step would be that the EXP then gets equally divided into the three pieces of gear that still need repairing.

What occurs is that the EXP still gets divided into four groups, which would then flow into the separate armor pieces and repair them. However, three of the groups are consumed to repair the armor that is still damaged and then the forth group that cannot be consumed by the fully repaired armor enters the player’s EXP bar instead of, as mentioned before, getting divided between the three pieces of armor that need repairing 

Because of the way this is done, the player has to remove armor once it is repaired to maximize the repairing for other pieces of gear, leaving the player vulnerable as they repair their tools and armor.
The changes I am proposing are changes to the way EXP gets distributed and then consumed by the mending enchantment on gear of various kinds, resulting in equal distribution across all tools or armor held by the player. Fully repaired gear would then stop receiving EXP, resulting in faster, easier repairing of all tools with the mending enchantment.

These changes (or similar changes resulting in a similar result)were implemented in the 1.16 update for the Java Edition of Minecraft.