We had changed the narrow name of Spanish (main/es.xml) Miércoles (Wednesday) from M to X for .
In the CLDR 24 Survey Tool cycle, the format/narrow name remained X but the stand-alone/narrow name was changed from X to M. This is bad – there is no reason for these to be different, and in Spanish the X is preferred for Miércoles because M is also the first letter of Martes (Tuesday).
There really should be only one copy of the narrow and short day names. The integration of CLDR 24 ST data added a copy of the narrow names to newly create the format/narrow section (and changed stand-alone/narrow/wed to M). The format/short and stand-alone/short day names were already duplicated before, and were modified in parallel in 24.