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Hi @Bibi_123 and @Racoon7 other than selling them at the Sunday Market, the only other reason I can think of is that your game was not saved properly after you played so the data was not retained.
Make sure you are logged in when you play.
I must admit that I've never really taken notice of how many are in my market so wouldn't know if any disappeared! I might keep better track of it now.
May be take some screenshots that show this is happening and so can report it if it happens again?