ARP 2600 VCA sections have been annoying synthesists since 1971. Typically, they need a good voltage WHACK to fully open, and that's been a thing forever. The two ways around that are to either open the VCA slightly, then start applying an EG per taste via the "LIN" input...or to use the VCA control input marked "EXPL". When the 2600's VCA gets an exponential envelope, then you get the right sort of result. It's not really a Behringer issue, unless you consider copying the original design TOO exactly to be an issue.
However, since it IS a known issue that's been "modded around" in past versions, a good synth tech should be able to apply the same mod to Uli's version. And there's other potential mods in there, although B. gave us a few of them (like VCO sync). But unfortunately, the 2600 is rather infamous for DC on its outputs, which is probably why the original ARP spec had that tough-to-open VCA to prevent DC from going out all the time when the VCA is outputting audio, but at the same time having it also capable of level control over CVs and modulation, which requires a DC-coupled VCA.
Yet another reason why the 2600 is perhaps THE teaching synth of all time. How many other patchables do you know that offer that flexibility + an opportunity to learn why you use exponential responses for audio and linear for modulation? 50 years on, and the 2600 STILL can strut its stuff!