R-style markdown cells are not properly parsed

[smathot]

When executing this script:

# %%
# # Markdown heading
#
# This is a Markdown cell.

print('Hello!')  # This is a code cell that generates the output cell below

# % output
# [1] "Hello!"
# 

The resulting html looks like this:

In other words, the Markdown cell is improperly parsed (possibly because empty lines are ignored, but this needs to be verified).