Well, the situation is this. 
I don't have a motorcycle and haven't had a Garmin (bike) GPS for over 10 years. 
I sometimes still use Basecamp as a tool to quickly check whether a GPX file is valid.
This is also effective for the many Garmin extensions with schematics. 
Basecamp imports compliant GPX files but rejects invalid GPX files without mercy. 
However, Basecamp does not report the cause. 
I also follow the Kurviger website to some extent. 
According to forum members, Basecamp rejects the new Zumo XT2 export.
This is a good reason to investigate. 
I use Notepad++ with the xml plug-in and the compare plug-in. 
Notepad++ makes it easy to check the gpx core.
Microsoft XML Notepad also checks the Garmin extensions. 
This tool can be installed in no time. 
@foolproof. It is definitely recommended. 

Although sometimes difficult, it is commendable that Basecamp is strict. 
It seemed very strange to me that Garmin Tread gpx would violate the rules. 

@jfheath.
Thanks for the additional information and especially for the demo gpx file. 
Very surprising.
- The result is file 1.
It lacks just about everything required for a compliant gpx1.1 file.
The file does not even import into fairly tolerant GPS apps.
The file also does not import into Microsoft XML Notepad.

- Action.
Added encoding utf 8.
Added creator (mandatory).
Applied the topografix and Garmin extension schema references. (desired) 
Removed Ansi to utf8 and the Trade Mark symbol in the Garmin Tread app text.
- Result is file 2_

The tolerant gps apps do import the gpx.
The strict Basecamp does not.

- Validation using Microsoft XML Notepad.
The error list shows you the (4) errors.
trp:AdvanturousLevel is therefore not allowed.
The Garmin extension has not been respected.
Garmin is violating its own schema here.

Strange, as Garmin was always exemplary in terms of gpx conformity. 



Translated with DeepL.com (free version)