Lenders are bracing for losses across most credit products, but mortgages stand out because the share of those loans in forbearance has started to creep up, while the portion of auto loans and credit cards in arrears has become negligible.

The key difference: the mortgage forbearance program is imposed by U.S. agencies that back the vast majority of housing debt and it does not require borrowers to show proof of hardship, industry sources said. That has made it difficult to tell who enrolled out of temporary need or extreme caution, versus who will never be able to resume payments.

"No one really knows how many of these people who are in forbearance are actually going to be able to recover, and how many of them are also going to go to serious delinquency," said Tim Mayopoulos, chief executive officer of Blend, a digital mortgage platform.