Hi,
So I'm now convinced the missing shadows is due to (in my case at least) the previous installation and maybe incomplete or incorrect uninstallation of a competitor's weather product. Reason for thinking is that uninstalling SMP unexpectedly left XP11 in a very broken state (please refer to this thread for identical error message and circumstances https://forums.x-pilot.com/forums/topic/17008-sun_glow_sockedpng-critical-error/). After this error I clicked Steam's Verify Files Integrity which identified 39 files needing replacement. After this, I reinstalled SMP and noticed some shadows, but they weren't right... when viewed from an external camera, they would be positioned kind of in front of the camera and move weirdly in a different plane to the ground, rather than be positioned and orientated as they should against the terrain.
At this point I decided there was unknown mess so trying to troubleshoot would take much longer than reinstall everything, so that's what I did. I copied my plugins folder to another HDD location so I could later quickly pull needed plugins back in easily, uninstalled SMP, RWC, ASXP and a couple of other bits such as OrbX scenery, then made sure to delete the X-Plane folder completely. Reinstalled X-Plane 11.50b11 from Steam, reinstalled SMP, ASXP and RWC and tested. Shadows all looked good now in several scenarios, so I spent the evening reinstalling everything which of course does take a long time. It's a big job.
Result is that as of this moment, everything's working fine, including with the OrbX TrueEarth England scenery, and hopefully it will keep working. :) I know reinstalling everything isn't the first path people might choose to head down because it really can be a big job (and my setup is relatively straight-forward) but sometimes when things are broken like this was, it can work out to be quicker than the alternative.
Hope this post helps others.