Question Strange FPS drops on static scene in Android device

Unity 2019.4.6 - Samsung Galaxy A8+. Shaders - Unity Mobile Bumped Diffuse (Albedo + Normal).
I run scene and it starts with 60fps. Once i'll get in the center of the map it remains the same - 60fps. If i walkaround the map, then fps sometimes drops up to 50 (since it's quite heavy map).

Problems starts, when i leave my phone alone and doing nothing. Just really nothing. It is lying on the table and strangely but something around one minute later my FPS dramatically drops up to 26.

The scene is absolutely static, no moving object, nothing.

What i'm doing wrong?

 

Does the device heat up? Could it be CPU throttling?

 

Nope. Main thing is that if i'm running or doing something on the scene, the FPS is not dropping

 

after FPS drop, if you start doing something, it goes back to 60?

 

take some time, like a few seconds (up to 20), but yeah. As a result fps goes up to 60.