The flash of the LEDs when the level is set is a consequence of the original design. It happens. As for a level change causing the VFD level to change and stay dim, it sounds like the level setting data stream coming back to the dimmer are either being corrupted, or you are suffering from an extended version of the original "dimmer glitch" problem, probably exacerbated by the extention. Look at what the AS4800 dimmer chip does/receives for clues as to what is happening.