I think I've fixed it. For anyone else dealing with this, it appears its the media foundation engine that's the culprit. to change it to directshow (which works):
To specify the default video engine
When importing a video file, SONAR will first attempt to use the default Media Foundation video engine. If the video format is not supported, SONAR will next attempt to use the legacy DirectShow video engine.
To specify the default video engine:
1.
Go to Edit > Preferences > Audio - Configuration File.
2.
Under Configuration Settings, click Edit Config File.
The Aud.ini file opens in the default Windows text editor.
3.
In Aud.ini, create a new [Video] section and add a new entry called VideoEngine and set its value to 0. For example:
[Video]
VideoEngine=0
Valid values are as follows:
0 = DirectShow engine.
1 = Media Foundation engine (default).
4.
Save Aud.ini and close the Windows text editor.
5.
Click Reload Config Settings to reload the current audio configuration settings from Aud.ini.