[Aurora Mysql] Reader Instance - Freeable memory is continuing drop even there is no connection or query

Hello, we are using a Aurora MySQL cluster with 1 writer instance and 1 reader instances. Recently, we created a new reader instance, but haven't setup any connection to this instance yet (means there is no query, no connection to this new created instance). We also setup custom endpoints pointed to each separately reader instances so that our application can connect to each reader instance separately.

Currently we are facing with the freeable memory of all reader instances are continuing to drop and are dropping at the same pace (even on new created reader instance). We are suffering from this issue and don't know why it was happen, there is no special thing or RDS traffic, connections. Do you have any idea why this situation is happen.