How do I use Xdebug?
The format shouldn't dependent on the Drupal version, most likely you're running Drupal 7 and 8 on different PHP version which may use different Xdebug.

Since XDebug 2.3, the format of cachegrind files changed and WinCacheGrind does not support this new Callgrind format yet. 

So you should use XHProf, or different software which may support it, depending on your operating system.

See also: Missing function names when opening CacheGrind file in WinCacheGrind.