Keep in mind that anyone who has access to the collaborative playlist will be able to make changes in there. However, this might be caused because sometimes, some content is unavailable or removed due to licensing reasons and this might affect the total number of songs in your playlist.
Since you have a lot of songs in that playlist, it's possible that some of them were unavailable for some days and came back. That'd explain why there are no records of recently added songs.
Let us know if this helps. Have a great day!
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