If am charging my phone with power bank, after the phone don full, de power bank will still be on and reducing, so where is the power going at that time, or what is it charging at that time. I noticed some power banks does that too. I use romoss 30,000mah

On recent phones, when you're charging, the phone is being powered by the charging source, while some other phones do this when the battery is full.
Instead of using the battery, it draw power from the charge source, whether plugged into the wall or from powerbank. So your phone was being powered by the powerbank while preserving your 100%.
So I feel your phone discharges even on idle, that's why it's draining the pb. I doubt it's from the pb sha.
You can try another phone to see if it's same

My Oraimo power bank goes off when my Samsung A30 is 100%