Then:
1) Kill it.
2) Set up critters with higher Stealth check results than its passive perception (11).
3) Hit the party from the sides/ rear/ above/ below. Bulettes, Ankhegs etc bursting from the ground. Wyverns etc flying in from above. Things coming at them from the sides. While its off scouting the SE, monsters come in from the NW.
I mean come on.
Thats not your players fault. Its the DMs. You designed the encounters knowing the Imp was there.
If your party are getting the jump on your monsters, its by your own design.
Dim light imposes disadvantage on perception checks. Thats a -5 right there. Plus only dedicated scouts get to apply passive perception (see the exploration rules). And even then, the range they get to apply that passive perception is up to the DM.
It cant fly everywhere at once. Its stealth is only +5. Even while invisible, thats far from assuring it cant be noticed. Heck the first thing it flies past with scent (advantage to smell the thing, even invisible) and it gets eaten.
Just kill it. Often. It takes time to resummon it. When your party are running against the clock to complete their task and have... oh wait.
Plus, the thing probably doesnt want to be sent off as an expendable thing to die. It's evil remember. Roleplay it. Have it screw with the party and not tell them, or otherwise subvert its masters orders (all while trying to lay the blame elsewhere).
No, your DMing style encourages this kind of behaviour from your group, and you have no counter to it (which is why they do it). Dont blame your players. If they nova strike or rely on one tactic above all othrs its because you let them.
Kill the Imp. Have it subvert its orders and lead the PCs into a trap for fun. Unless given incredibly complex orders, it's always going to have wiggle room to do something mischevious. And the more complex the orders, the more loopholes you should be able to find. Its a devil for gods sake.
Of course, sometimes it should locate the ambush and bail out the party. Its a class feature, and the Warlock should be rewarded for using it.
Its just your job as DM to not let that happen all the time.
Look man, try me.
Ive offered a billion times, but there is no party (optimised or otherwise) I couldnt run a game for. Feel free to present to me a party, and I'll happily stat it up 6-8 medium-hard encounters (and a reason to engage in those encounters) to challenge the crap out of it.
I use the same rules that you do, but our DMing styles differ. Thats why we get different results playing the exact same game. Its also why my advice to you is generally 'DM differently' instead of 'Invent a whole slew of house rules to deal with the problem; a problem you probably created yourself as DM'.