Random suggestion: if you use the Open App action in Shortcuts, is Safari there? And regardless, what happens if you use the Open URL action? That might kick things into place.
Thanks all. @RosemaryOrchard ’s suggestion sees a “Safari is restricted” message come up, and @JohnAtl provided the solution! No idea how that happened!
Edit: For some bizarre reason, my wife’s iPhone has picked up my son’s Screen-time restrictions. My son and daughter are linked to my account, and I manage their screen time from my phone. My wife has her own, completely separate iCloud/AppleID account — so no idea how that came to be. But oh well, sorted now.