Hi @Bhup, welcome to our forum!
Due to a change in Google's privacy policy, implemented on January 7, 2019, the favourites tab in the Phone app, and the shortcut menu, will consist of your favourite (star) marked contacts, together with frequently called contacts based on one day call history. This means that a previously frequent contact, but not within the latest day, will not be listed.
Make sure to mark a contact as favourite, to guarantee that this contact is always displayed in the favourites tab and in the shortcut menu.
I hope that answers your question as to why they are removed on a daily basis.
