I analyzed the entire library through Mixed in Key, but noticed that some keys don't show up in the MIXO playlist. I realized that only specific keys are not displayed, while the key itself is visible in the edit window.

Check these keys
Ab Bb, Gb, Eb, Db
Abm, Bbm, Gbm, Ebm, Dbm

As you can see, these 8 keys are not displayed. They even match in letter .