The fact that a Labour held seat voted Leave does not imply that most Labour voters in that constituency voted Leave in the referendum. That Leave vote was made of people who had voted for the whole range of parties.

There have been a number of researchers who have found that it appears that most people who had voted Labour in 2015 voted to Remain in the referendum, whereas most Tory voters went for Leave.
Here is a site that includes a breakdown of how the refrendum vote was made up.

For 2015 Labour voters it's given as 63% Remain, for Tories it's only 42%. And many of those Remain voters were probably at least as much influenced by a wish to hit out at the Cameron government as by the actual issue of Brexit.   When it came to the 2017 election they voted Labour again.

That's not to deny that there were many Labour voters who went for Remain - but the assumption that all the Labour leave-voting constituencies are full of people ready to turn there back on the hope of a Labour government at any sign of readiness to allow people to have a second chance to decide their future is very questionable.