Bank asking for original deed to sanction loan is this justified

Please refer to the following issue : Mr A purchased a new flat, sold it to Mr B after a few years and I am buying it from Mr B. Mr B purchased from Mr A around 20 years back. I am taking a home loan for the said purchase from a leading house who have sanctioned the loan but are raising issues before disbursement. They want to see the original deed between the builder and Mr A, which is not available with Mr B. Without this they are unwilling to disburse the loan. Is this justified ? If not, how can I challenge them ? Thanks