This seems similar to and yet different from other threads here related to WordPress and getting a Drafts Document to load/work properly in the WordPress/Jetpack app. I will do my best to explain, and apologize in advance if I am unclear.
When I use the Sharing → Jetpack Save as Draft menu item from other apps (examples: iA Writer or the Files app) with an exported markdown file created in Drafts, what gets created in Jetpack is a draft post with Markdown rendering as expected.
But when I use the Share action from within the Drafts Basic action group and try using Jetpack Save as Draft, what gets created in Jetpack is a draft post with no Markdown rendering at all: just the raw text with all the “Markdown markup” still intact. For some reason, Jetpack doesn’t recognize this draft as being Markdown at all. I tinkered around with the Share action in the Basic action group for a while, trying various combinations of template tags besides/with/in place of the default of [[draft]] but could not get a working configuration which gave the same results as the Share → Jetpack Save as Draft item in other apps. That’s the “bad” news.
The good news is that I located a different way to get a Drafts document into Jetpack that works correctly from within Drafts itself, and this workaround is plenty good for me 
Steps: In the workspace draft list, tap and hold the draft you want to upload to Jetpack. Then, from the context menu, do Export… > …As Text… This opens the System sharing menu directly (rather than through a Drafts action). Select Jetpack Save as Draft, and things should at this point “just work.”
I hope this helps others get their writing into Jetpack more easily!