There’s a couple of things going on as I see it. The first thing is that Solo in Cubase actually Mutes all other tracks. The difference may seem like semantics at first, but there is a subtle difference and pre fader sends puts a spotlight on that difference.
The second thing, as you found out, is that Muting an Instrument channel in the mixer also mutes the MIDI. I find this behavior very counterintuitive. If you use Rack Instruments, you can actually mute the MIDI Track and Audio Channel individually but Solo always engages on both(!).
Luckily there is—the Listen bus. It does require you to setup and use Control Room but IMHO, you should do that anyway. Once Control Room is activated, click this “L” button to listen to a channel with sidechain routing in place:
