Mentioned in MDL-68608, often you set up a MUC default store mapping to say redis, but in reality the mapped store ends up as the default cache ignoring the default mapping.
I've seen this in the past with any cache type, and thought it may have been related to MDL-68440.
has a non-specific relationship to
MDL-68440The MUC cache admin page regularly reports the wrong number of mapped caches
Closed
MDL-68608Improve the readonly session debugging message