I've made several tests. It seems I found 1 specific configuration that does the trick. Drawback: 2 additional Volume Blocks needed.
Application: Blending gradually 'Dry Mono Path A' from 0% to 100% 'Wet Stereo Path B' without volume change at any position EXP Pedal
1. Set Split Block to either Split Y (Balance A and B = Center) or Split A/B (= Even Split) - no controller assign(s)
2. Set Mixer Block to default settings: Level A = 0.0dB, Pan A = Center, Level B = 0.0dB, Pan B = Center, B Polarity = Normal, Level = 0.0dB - no controller assign(s)
3. Insert a Mono Volume Block at the end of Dry Mono Path A just right before Mixer Block. Assign EXP Pedal to Position (Min Position = 100% / Max Postion = 0% / Curve = Linear)
4. Insert a Stereo Volume Block at the end (after all Wet Effects) of Wet Stereo Path B just right before Mixer Block. Assign EXP Pedal to Position (Min Postion = 0% / Max Position = 100% / Curve = Linear)