As part of the
Index selection is based on continuous keys only in 6.6.x. (i.e. same as pre 6.5.0)
Make Index selection is based on continuous keys only and don't include skip keys.
Once Index selection is made skip keys are included in span generation.
This decision is made to fix regressions. This has been addressed in 7.0 and backport is too risky.
Query can add another predicate or USE INDEX is alternative.