The ASC1 has a remote On/OFF switch on the front panel that is used on systems controlled by the STB on most systems to prevent identical IR commands from also controlling the ASC1.
Since your system is configured for only manual ASC1 positioning, the ASC1's Remote ON/OFF switch would disable the ASC1 RCU operation and positioning could only be controlled via the ASC1 front panel or to enable the automatic STB control via DiSEqC 1.2 commands.
In your case there may be an easy fix if you wish to keep manual ASC1 positioning with ASC1 RCU. Your VU+ STB image must use a RCU (Remote Control Unit ) key mapping for the same IR code used by the ASC1. Key mapping can be changed in Enigma2 images. One example in TNAP for the OS Mio: Menu>>> Setup>>> System>>> Expert Settings>>> Hotkey Setup.
You might instead map the Channel Up/Down key assignments to different RCU keys.