BigQuery bugs out when you send logging data that has unknown data structure. We add a fields to the 'Fields' data that is variables that are passed in to the various ThrowUserError or ThrowCodeError exceptions in the Bugzilla code. The $vars hash is converted to key/value pairs in 'Fields' such as "var_$key": "$value". Problem is $value can be an array reference, hash reference, or just a string. BigQuery expects all fields to match a schema and there is not way to say, allow anything.
I propose changing to just passing a single field called 'vars' whose value is a JSON string version of the $vars data.