That's just the way it works, bud. Zen flute is for regen tanks like naoe and hohjo ujiyasu.

I strongly recommend against using souls on soul tickets. There's lots of things to spend soul on now that does better for you in the long run. researches are always worth it. always.

Uni evos are currently the best way to spend soul - assuming you've got the parts from the fire dungeon. Which, btw, god damn i love this event. i 4*'d my UR sasaki, 4*'d my halloween kenshin, about to 4* my sanada yukitaka. and i have enough parts to build 5 more when i get the soul for it.