We believe that there is a bug in the course import that affects Redwood, Sumac, and master. In short: it seems that for any custom XBlock, Studio will import the block if it is represented inline within the same file as the <vertical>, but it will not import the block if it is represnted in its own file course/<BLOCK_TYPE>/<URL_NAME>.xml. The expected behavior is that both syntaxes would work for all blocks, whether built-in or custom.
We will fix the bug on master (and for Teak), but we’d also like to figure out exactly when the bug was introduced and why, and backport fixes if possible. If you use course exports with custom XBlocks, it would be helpful to hear if your site experiences this issue. If you reply, let us know which release you are running. Thanks!