My 2018 +4 would do the same if I did not use an Optimate. Other chargers are available. I don't leave the car more than 2 weeks without it being on charge. It might last 3 to 4 weeks but it's not worth the risk. I have a lead wired in so it's easy to put on charge.
The alarm system takes power all the time.
Cheers
John.
