From Beyond Compare 5.0 Beta - Known Issues:
- Windows 10: Shell extension isn't enabled in non-Explorer apps.
So it seems like an issue on their side, not just affecting Opus.
There are also much nicer ways to use Beyond Compare in Opus:
- "Smart" front end for Beyond Compare
- "Smart" front end for Beyond Compare - #46 by Leo (my personal fork)
- External Compare and Merge Tools 2.0 (handles BC and many other tools)