They can't do that. Think about it. How can they distinguish between bad luck like yours and someone just unplugging their router to leave a game?
Your unstable connection caused a bad experience for 11 other people, the ban is justified.
heavy melee cancel is, as of now, part of the game. It may be changed in the future but is a known mechanic and the devs have shown no interest in removing it.
indeed, it has already been changed without being outright removed.
You do not get banned at all if this is not something happening regularly.
You are correctly being assessed bans. If you do not have a reliable connection, you are ruining the game for 11 other people.
This is known and seems to have been intentionally added awhile back as a part of the ability to mantle glide.
https://www.dlmovement.wiki/doku.php/universaltech/mantle_glide
https://www.dlmovement.wiki/doku.php?id=universaltech:sliding#slide_storage
^ see "slide storage"