I got the feeling that they dig foliage once in their lives and don't do it anymore, I spent a lot of time at AFK and she never dug foliage a second time, maybe the interval is too long or how does it work?

Took a look at the code and it seems like the foraging timer is never decremented, meaning it never counts down at all. The timer also isn't saved to NBT so it resets on world reload, meaning logging in and out instantly makes the sugar glider forage then never again until the world is reloaded again.