Ordering in firestore


I have a record collection of thousands and would like to know how to index them correctly according to the time stamp.

there is. timestamp field I have and I have set it up link below. The problem is that when I pull 20 records from the collection it works and shows from the 01 Dec till today but the second time I pull 20 records some dates are from the today and others are from the 1st etc. It doesn’t appear that the indexing in descending order is working.

I am not sure If I am doing something incorrectly.

Please let me know if there is a step I need to look at.