Recently, some Android Auto users have found that voice commands haven't been working quite as they should, but it appears that a fix for the problem is arriving in an update now.
As reported by 9to5Google, the problem is that Android Auto was passing voice commands through to Google Maps only, rather than to a user's preferred navigation app (such as Waze). If you already have a navigation app running, Google Assistant is supposed to send your Android Auto voice commands to that app (and if one isn't running, they should go the one you last used).
However, it appears that Google Maps has been hijacking these commands, with all requests steered to the 'default' app. Apparently, it's been possible to get around this by totally disabling Google Maps on your phone, but that's not exactly ideal – just because you find Waze (or another app) to be preferable for driving in your area doesn't stop you from preferring Google Maps for walking directions or finding the nearest burger joint.
Happily, 9to5Google reports that some users are finding that the version 15.13.46.28 update for the Google app is pointing Google Assistant back in the right direction, and making things work how they're supposed to, again.
Despite being owned by Google, Waze has some great features that mean people prefer it over Google Maps – such as its useful new driving alerts – and there's a similar story for using other driving apps that should support voice commands. Having confusion over which app is directing you partway through a journey is, shall we say, not exactly ideal – so it's good that a fix is coming pretty quickly for this problem.