Most probably that's because of the loss of OpenGL context and the way Android handles Activity finish (process/task of the app may not be destroyed when main activity is closed).
Check out this Stackoverflow answer for more details:
You have rediscovered the mismatch between the lifetime of Java
objects (tied to the life of the application process) and the lifetime
of texture objects (tied to the life of the OpenGL context which is
tied to the visibility of the Activity).
On app "exit", just the Activity is exited, and Android is caching the
process in the background. When you "restart" the app Android just
starts a new Activity in the same process. In this case the Activity
is finding a valid Java Texture object, but the underlying bytes it
"points to" in the OpenGL context are gone (since the OpenGL context
is invalidated when the Activity is no longer visible).
The fix is to re-load textures on activity creation. You must make
sure all your objects that contain textures (and objects that contain
objects that contain textures, etc) are tied to the Activity
lifecycle. Generally this means avoiding static variables (which are
part of the application lifecycle), but you can jump through hoops to
invalidate and re-initialize globals if you want.