I love rereading to catch this kind of thing! It gives more depth to the story for sure. I’ll certainly be rereading this one once it’s complete. It’s so good
actually, i think that the memory erasure didn't work because back when tatsumi was getting accustomed to his wave voice, he unintentionally uses it on kazushi by saying that he vows to never use his power on him. so years later, when he tries to use it to erase kazushi's memories of him, it doesn't work because of what he said in the past. kazushi probably passes out because of the strength of tatsumi's voice
...and reading everything once again, it's clear now Σ(っ°Д °;)っ
http://www.mangago.zone/read-manga/see_you_later_mermaid/uu/ibx_chapter-1/pg-11/
http://www.mangago.zone/read-manga/see_you_later_mermaid/uu/ibx_chapter-1/pg-12/ => I thought it was strange (and even ridiculous) at first, like how could some sand get in his eyes though? but, actually Kazushi was CRYING HARD & UNCONTROLLABLY because he got to meet Tatsumi again! and now I'm teary??
About why Kazushi did not forget his memories with Tatsumi, I think this explains: http://www.mangago.zone/read-manga/see_you_later_mermaid/uu/ibx_chapter-2/pg-28/
Tatsumi couldn't eliminate the root cause (Kazushi's love for him), and I think his power did affect Kazushi (he passed out right?) but only for a few hours because Kazushi's feeling towards Tatsumi was so strong that the effect wore off so soon??
Anyway I love this manga much more than I had expected. Thank you very much dear translator & uploader (づ￣ ³￣)づ