Well, the actual autopilot is doing whatever you want, mostly. You just need to tell him what you want.
You’re aware of the fact, that not all Aircrafts are equipped with Autothrust?
I’m sorry to say, but if I do understand you correct, there is no bug except in front of the monitor…maybe…. 
And another thing. Switch off all that ■■■■ AI Assistance. It’s not worth to tell it AI and is responsible for a lot of problems like yours.