I ran into same issue when doing a vBulletin bulk import.
Looking into source code there is a way to blacklist selected tags. Here is what you are looking for if you want to blacklist/ignore [quote] tags:
xyz.bbcode_to_md(false, {}, :disable, :quote)
I applied the above fix to import script base libraries as per:
https://github.com/discourse/discourse/commit/71a5369fef33f2d8d3b1d5420a5a6e775c439b0d