C++20 replaces the individual <,<=,>,>= operators of std::vector (and of many other standard classes) with a single <=>.
Internally it tries to use <=> to compare the elements, and falls back to the old operators if the type doesn't overload <=>.
Since you have a non-explicit operator bool, applying <=> converts both operands to bool and compares those. The fix is to make operator boolexplicit (which is a good idea in general) (so that <=> fails and vector falls back to the old operators), and/or replace <,<=,>,>= with <=> (which is also a good idea in general).