Thanks to the usage of third party methods, i was able to combine blast protection and the normal protection enchantments in a single armor piece inside of my own local world; problem is, i was wondering why the blast projection wasn't changing at all by upgrading its level, until i realized that what caused this to malfunction was the combination of protection enchantment themselves. Now- i know that technically this isn't an officially supported function and therefore mistakes are bound to happen, but since i'm a hosted server owner who makes tons of custom items like these, it frustates me to see that i cannot get it to work because of this. I would be forever grateful if this could be fixed, so that i continue creating content for my server without having to rely on work-arounds. I'm pretty sure there's been a version of java where you could merge the enchantments just fine with no problems; that feature has since been removed, but i don't see why this must be an occurrence on bedrock if the enchantments seem to have worked as intended on Java.

How i reproduced this:
-first of all, i wore the first enchanted armor with blast protection I and protection IV, then i gave myself explosion damage via the /damage command; and as expected, i received less damage than usual due to the blast protection.
-after that, i tried switching over the second set of armor with blast protection III and protection IV then tried again: to my surprise, the amount of damage i took was always exactly the same, no matter the level of the enchantment.
-lastly, i made a brand new set of armor and repeated the same exact process with just the blast protection enchantment, and the levels actually scaled the damage correspondingly, thus i figured it must have been the mix of protection enchantments.

Expected results:
The damage received should increase for each level of enchantment added to the armor, regardless if it is mixed with other protection enchantments or not. Instead, the multiplier remains stationary and doesn't seem to change at all due to a different kind of protection being in that same armor piece.

Most of the steps that have seen listed here are also showcased in the video provided below, so hopefully it's enough evidence provided.