Pulling the rug out over tone is not kind to your audience. If you tag it as fluff the ending should be fluff, if you don't want to spoil tone then just don't tag tone at all.

Likewise, if the character we see a bunch ends up dead then you need MCD or Choose not to Warn. You want a twist that's fine, but don't actively lie in the tags and warnings.

Some of us lost family members and purposefully mistagging by saying it will be fluffy/happy can wreck a person. How would someone who lost a partner feel running into this if you tagged it as happy relationship fluff? Omitting stuff is one thing, but actively misleading readers through the tags is not cool.