Gaining altitude in an unpowered plane

How to reproduce:
0. I use a slightly modified tutorial airplane with solid fuel rocket, but I think any airplane should work
1. Fly and reach around 8 km ASL.
2. Make sure the airplane is not propelled forward with anything (unpowered)
3. Set Angle-of-Attack to 6°
4. Notice the airplane gliding and losing altitude before it stops losing and starts gaining altitude relative to sea level.
5. Every once in a while, the airplane mysteriously stumbles and loses altitude again, but later it'll start gaining altitude again as explained in step 4.

What happens: The airplane keeps gaining altitude without power.
What should happen: The airplane should lose altitude without power.