Since upgrading to 8.5.1 user/login, user/logout & user/register are 404's

Since the upgrade to 8.5.1 (using composer and drush @sites updb) the user/login, user/logout & user/register are all displaying as 404 pages. 

Here’s a stack exchange about the same problem https://drupal.stackexchange.com/questions/258805/how-to-debug-404-on-user-login-user-register-user-logout that I am not alone in having, there seems to be more than 5 others that have the same issue.

I’ve tried different versions of php, in case of symphony needing it for routing, 7.0.22 (as on server) and 7.1.10 (local dev apple machine), by expressly defining them in the composer.json, with no success. 

I’ve written a small module to check the routeProvider details for user.login, user.logout, user.register and they are an exact match for the settings shown on a new install of 8.5.1, except for 3 lines:

user.login
upgraded version:
[regex:SymfonyComponentRoutingCompiledRoute:private] => #^/user/logout$#sD

new install:
[regex:SymfonyComponentRoutingCompiledRoute:private] => #^/user/logout$#s

user.logout
upgraded version:
[regex:SymfonyComponentRoutingCompiledRoute:private] => #^/user/logout$#sD

new install:
[regex:SymfonyComponentRoutingCompiledRoute:private] => #^/user/logout$#s

user.register
upgraded version:
[regex:SymfonyComponentRoutingCompiledRoute:private] => #^/user/register$#sD

new install:
[regex:SymfonyComponentRoutingCompiledRoute:private] => #^/user/register$#s

The difference being an extra D at the end of the regex.

code used:

$router = Drupal::service('router.route_provider');
    
print_r($router->getRoutesByNames( ['user.login','user.logout', 'user.register'] ) );
    

I’ve also tried putting debug code in UserAuthenticationController.php and UserLoginForm.php to see if drupal is trying to load them at all, and no it is not. 

Downgrading on local dev machine to 4.6.4 causes the login, logout and register page to work again. ( but cannot be done in production due to too much new content for db replacement, and new errors if the db is not replaced ) 

Any help to debug/fix this would be amazing.  I’m running out of ideas on what to do. 

Drupal version: 


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