Unfortunately, it is not a problem with the middle C. In fact, it is a problem with the VST3 plugin, Studio one and Synchron WW.
The VST2 plugin works fine in S1 with WW. VST3 plugin and all the other Synchron libraries (String pro, Elite, Brass, Strings FX) work fine in S1.
Also, VST3 and Synchron WW work fine in Cubase 11. VSL, could you please test the VST3 plugin with S1 pro and the Synchron Woodwinds, a low patch (bassoon or contrabassoon), notes C1 to F1 ?
Thank you very much.