Hohige
Explorer
Imperceptible spells means imperceptible.All the imperceptible casting means is the victim does not know your location. It does not mean any other rules of combat, such as rolling initiative, are abrogated.
The target doesnt know nothing.
Read Xanathar's rulesAgain: Subtle Spell doesn't make the spell imperceptible. It makes casting the spell imperceptible. [EDIT: It at best makes casting the spell imperceptible. If there are material components involved, you still need to deal with them, which might enable someone to know you've been casting.]
If (in the absence of some mass telepathy effect) you use Subtle Spell to cast Mass Suggestion, everyone can hear the words you say--there's just not any abaracadabra ala kazam stuff before it. If some of them save, they might notice their companions behaving strangely, and be able to figure out what happened.
I'm pretty sure that example is entirely consistent with what's in Xanathar's and with the relevant SA stuff.
If you use Subtle Spell to cast Mind Spike, it doesn't give away your position, but the victim knows they've taken psychic damage, and can reasonably conclude they're under attack. A ruling otherwise is extremely friendly to the party and the sorcerer.
It's no wonder your opinions of your characters are so ... skewed, and why they flop so persistently when faced with the actual rules as the game is actually played (at tables other than your own, which seems to have gone pretty far astray--but if y'all are having fun, that's the primary goal, it's not badwrongfun or anything, there are just some presumptions baked into the game y'all are playing and D&D 5E that make them very different games in practice).
Its totally imperceptible.
Again.
Imperceptible.