Hi Sonali Rout,
As Ludwig mentioned; you can use currency revaluation for bank in the Cash and bank management module itself, without processing it in GL.
However; you did not mention clearly if you are actually following this or not. Looking at your query; it looks like, you are doing currency revaluation for subledger 'Bank' at main accounts level only. Also; you did not mention rate from which date in 2019 you are using for revaluation. I hope you are using rate as on 31st Dec considering the revaluation period Jan to Dec as you mentioned.
If this is only in main account level; then check as per the following.
--- Open Ledger settlements form in GL periodic tasks. Check if you have settled any lines here. If not, then system will revalue all these as available 'Open transactions' from Jan01 to Dec31 (date ranges as you highlighted).
For example; in below screen print; you can see nothing is settled yet in Ledger settlements form.

--- Now identify the account for which you have got huge revaluation amount recorded in ERP. That account must have multiple records posted on different dates which is not yet settled. Therefore; system will revalue all those lines one by one.
--- Chose any main account + dimension combination as you mentioned and filter it in Ledger Settlements form.
--- Now revalue each line of this combination with 31st Dec rate and check what is the difference in value you are getting between the amount in local currency (as showing in Ledger settlements form) and amount in local currency after revaluation made.
--- At the end; do a Total of all these difference amount. Hopefully you will be able to validate the reason for the huge amount recorded during GL revaluation.
You may have to spend some time for this activity to reach the conclusion because revaluation is a very straight-forward approach and it will only consider open transactions to revalue on a given date ranges based on certain rate
Kindly check and update.
Best regards,
Sourav Dam
Kindly mark this thread 'Yes' if this is answered your query which may help other community members in this forum.