Filed as [479][].
Funny that this has popped up because the only change that I can find to CLOS/MOP between 1.7.1 and 1.8.0 was in the CALL-NEXT-METHOD/NEXT-METHOD-P implementation. But this is why we test. Or should test better.
Thanks for the explicit report with the test case!
[479]: https://abcl.org/trac/ticket/479