This is the way collections work and is not a bug. A collection brings together data from more than 1 account. In the case you are describing you must have a balance in another account on the date in question that is also included in the collection.
For example, imagine a collection called "House Equity" which has the accounts "House Value" and "Mortgage" as members. To start with let's also assume that balances exist for both accounts on 8 Dec 2019, 15 Dec 2019 and 14 Jan 2020. The collection will show a balance on all 3 dates : the sum of the balances of each account on these dates. Now assume you deleted the balance on 15 Dec for the Mortgage account only. The collection will still show a balance on 15 Dec : the sum of the balance on 15 Dec for House Value and the previous balance for Mortgage (the one from 8 Dec in this instance). You would need to also delete the balance in House Value on the 15 Dec for that date to disappear in the Collection view.
Image by Gerd Altmann from Pixabay