Using 2 tables (A & B), joinable by their PK, with the PRIMARY KEY of A being sorted DESC (PK in descending order as it's allowed by MySQL 8), and A having 2 indexes in ASC ordering.
When I perform a query like
SELECT * FROM `A` a LEFT JOIN `B` b ON a.id <=> b.id WHERE a.idx1 = 'X' AND a.idx2 = 'Y';
It will return nothing... BUT, if I add a precise condition like:
SELECT * FROM `A` a LEFT JOIN `B` b ON a.id <=> b.id WHERE a.idx1 = 'X' AND a.idx2 = 'Y' AND a.id = 1337;
It will find the row with the specified value as PRIMARY KEY.
An explain show that MySQL will use 'index_merge_intersection' optimization when it doesn't find anything. If we disable the 'index_merge' optimizations, the first query will return all results as expected.