I suspect I see the issue. Try adding UseStreaming(false) after calling Query(), and see if that affects it.
Linq2Couchbase 1.3 turned on the new streaming query result function by default, which is a more memory-efficient way of processing large query results that was added recently to the main SDK. However, the streaming function in the main SDK doesn't support custom serializers, it's built around Newtonsoft's JSON reader.
If this is the issue, we'll probably need to add a more user-friendly way to disable streaming in LINQ globally for custom serializers.