When wearing parts of the Diving set in conjunction with armor pieces from other armor sets, the armor value the player will see on their HUD will be inaccurate. Testing with the /damage command, this doesn't seem to be only visual, and to my knowledge it is not stated that the full Diving set must be worn to receive the listed armor values.

An example scenario to replicate:

• Equip a full set of Diving armor. The player will have 15 armor (7.5x HUD icons) in total, as expected.
• Swap out the Diving Leggings for a pair of Iron Leggings, which provides +5 armor compared to the Diving Leggings +6 armor.
• The players armor value will drop to 9 armor, instead of the expected 14 armor.
• Interestingly, swapping the Iron Leggings out for Diving Leggings again will bring the armor HUD up to 10 armor, as the Diving Leggings have an extra point of armor. However, upon taking damage, the player's armor will reset back to 15 after the damage is dealt. This delayed armor value update does not happen if the player does not swap the Iron Leggings out for Diving Leggings again - the player will remain at 9 armor with a Diving Helmet, Chestplate, and Boots, with an Iron Leggings.
• This incorrect armor value even occurs if the player starts with no armor, and goes straight to wearing a Diving Helmet, Chestplate, and Boots, with an Iron Leggings.

No clue what causes this. It only seems to be Diving gear that is affected, as well - the Hazmat, Primordial, and Darkness sets mix and match just fine.