Group Policies for Firefox can bypass Tor Browser's proxy settings
Assuming your Windows environment has a Firefox group policy (GPO) that specifies e.g. using system proxy settings then Tor Browser happily follows that and is ignoring its own proxy settings without notifying users.
What should actually happen is that Tor Browser is ignoring those Firefox GPO settings instead.
This got tested with Tor Browser 8.0.8 on Win10 1709.
Thanks to Kit Chung for this report.