After sync of iTunes library to iPad (multiple library scenario), iPad shows "mixed" library

Since 2 or 3 weeks now I face an absolutely confusing situation in regards to my music on my iPad. First let me describe my specifc scenario:

Environment = Windows / iPhone 13 Pro / iPad 5th generation

Multiple iTunes libraries (to be precise: two):

• One containing my "private" music. Usually none of my devices is connected and thus neither synchronized "physically" (via USB connection and iTunes sync) with that library. The music is synced to (only) my iPhone via iCloud.
• The second library is causing confusion: It contains my Ballroom Dancing music which I need a a professional ballroom dancing educator. This library is updated on a regular basis by connected my iPad to my Windows PC when iTunes is open using that library.

This worked perfectly for years up to as said around 2-3 weeks. During the regular update of new dance music to my iPad (second library; USB connected to my Windows PC), I was wondering that new playlist were not synced (well, different problem, but not the point here). Instead, the music library on my iPad suddenly now contained a confusing mixture of both libraries. New music titles were actually synced from the second library (but no new playlists) but the entire (I guess it is the entire) first "private" library showed up on my iPad (titles and playlists). Today I was actually editing the first library in iTunes on my Windows PC, adding some new titles and suddenly, all of my dance music of the second library appeared. In other words: the first library in iTunes now looks like the music library on my iPad. However the second library in iTunes still looks correct and only contains the dance sports music. Just by chance I checked the library on my iPhone while writing this text and also here, I see this "mixed up" library of private and dance sports music.

This is somewhat completely confusing, I did not change settings in Apple Music nor iTunes Match (I would not even know how to do so).

What could be the reason for this strange syncing? What do I have to do to achieve the original situation again:

• One "normal" library syncing with Apple Music / iTunes Match to "all" my devices except one (the iPad).
• One library not syncing from iTunes with Apple Music / iTunes Match but only syncing with my iPad when connected with my Windows PC via USB; and vice versa the iPad NOT syncing via Apple Music / iTunes Match.