If someone uses boosts, and still loses a match, they should get the boosts returned.
They’ve still lost. They lose any applicable points, and their heroes are hurt. In PvP, they can’t redo that battle – they’ve lost.
The game can glitch in various ways – a glitch loss that steals boosts is a bad experience.
If the game only consumes the boosts on a win, that would make weird losses less frustrating.
Potentially abusable. You realize you’re in a tight spot and can lose, while still having all your heroes still alive? Retreat, heal, get back without losing the boosts. In PvP you’d still lose points, but in PvE it would be abused to hell and back.
A trick that involves massively harming your characters for zero gain won’t be abused all that much. It could be abused a few times. If I’ve boosted to the eyeballs against a level 390 Daken, and the opening board is half green, I might slink away and spend a few minutes healing them back up in the prologue. But I’ve gained nothing and lost time – this wouldn’t be a common abuse.