WP code snippets showing in Tumblr version

You mean shortcodes? Provide a link to an example of where this happens, please, there’s not enough information in this question to understand what the problem is.

Yes, shortcodes.

https://iammarkmanson.tumblr.com/

The shortcodes I tested work fine. This is a problem with other plugins providing the shortcode, not with Tumblr Crosspostr.