"Full spring" and "Serenade of the nether" both have a problem of the blocks not flowing when created, and I may have found an answer. In the code, it looks like you are doing the equivalent of "/setblock X Y Z minecraft:water" and "/setblock X Y Z minecraft:lava" command. As it turns out, to have it start flowing it needs to be "/setblock X Y Z minecraft:flowing_water" and "/setblock X Y Z minecraft:flowing_lava". If your code works like my commands, this should in theory fix the little bug that has, well, been bugging me.

Those blocks are not source blocks and can't be used for anything except burning up items

I have tested that, and they can in fact be scooped up in a bucket and will not disappear after a block update.

Just updating the block would also do the trick. Plus, changing that would a noticeable hurt in performance especially with lava so I recommend not to.