There is a bug in Cubase’s PDC engine when it comes to Sidechains. I have verified this now with every single plugin i possess that is vst 3 with sidechain inputs, that also has latency.
Basically, if the plugin you are using to receive the sidechain, has latency itself, it can’t compensate. For example, you have an audio track with a pad and insert a latent gate on it. You feed this gate a drum loop as it’s sidechain. It will be rhythmically out of time. If you use a zero latency gate however, there is no issue.
Also, you can put even plugins with massive delays before or after the sidechain plugin, and there is no issue. Only the sidechain plugin itself can not have latency.
I also checked that said plugins are reporting their latency correctly to cubase, and in all cases they were. I do this very easily by duplicating a track, putting the plugin on one but not on the other, and flipping phase on one. Null means plugin is reporting latency perfectly:)
If you want a really easy way to see the issue, just try for example, plug in alliance G8, and set the look ahead to 50 or 100 milliseconds, then sidechain something into it. presto.
Now, because Cubase has an exhaustive PDC engine that is usually top of the heap, i really do not think this is by design, but rather a bug.
I mean even logic, whose PDC is very flawed, does this particular thing perfectly.
If any further explanation is needed, please ask me and i’ll try clarify more. Cheers!