Why does my cassette keep coming loose?

I’d say the thread has lost some of its newness? Hopefully on the cassette not the free hub, got an old cassette you can nick the lock nut off of?

Tried some loctite? Something like 242 might help or 243 is a wee bit stronger

Missing spacer?

Seen similar on a couple of  DTswiss freehubs where the last cog doesn't sit fully on the end of the freehub. Overhangs a bit much. It then bites into the freehub. This can let it move, which loosens the lock ring.

Check the end of your freehub see if it looks a little chewed up.

If so. New freehub time...

the shop tightened it for me (bastard tight with a massive lever).

Any suggestions other than do it up tighter?

Given ‘… massive lever’ tightness may not be the problem.

While Loctite is a solution for many things a quick-loosening cassette lock ring seems like a thing to find the underlying cause of before a Loctite solution.

I’d go with mangled threads or a missing spacer.

Missing spacer would be my first diagnosis to explore as mangled threads would have gotten in the way of the shop’s
work.

Good luck.

Thanks all!

I don't think it's the spacer, when I had this in the past I checked this carefully.

I'll check the threads.

The free hub is chewed up to the extent I have a spare one ready to go on. Maybe it's time to swap it in

I would swap it and perhaps use a new lockring if possible, any freehub that's working correctly shouldn't be coming loose.

I had this issue and changed out a metal spacer behind the cassette for a plastic one.

A different occasion I changed the locknut and it seemed to work.

No idea why, really, in either case. But I agree with the comments above to try these first.

If it was a spacer issue surely it would be obviously moving, I regularly check for play by giving the cassette's big cog a wiggle, it's a habit I do after installing a cassette too so I would instantly know about any play, It sounds like a worn or sloppy thread issue, especially if overtightened, 40nm is decent amount but can be achieved with a regular length socket wrench, bastard tight with massive lever suggests way way over 40nm, new freehub and lockring time, threadlock might cause a huge removal issue, I wouldn't.