Hi @bielik01 apologies for the late response.
I have checked and according to the development team this is not an issue and is working as intended.
A settlement pack consists of two parts in the store (tree and decorations) which makes a total of 5 items (1 tree and 4 decorations). Whenever you purchase the tree from the settlement pack (either from Reda or from the property tab in the store), there will be 4 decorations left from the settlement pack, which is basically the last part of the settlement pack. The rule for that is when there is only one part left ...