The SimpleMDE editor has GFM activated by default [1] which means that it does autolinks [2]. The problem is that markdown2 does not. So user put raw links thinking they will be rendered as link (because of the SimpleMDE preview).
I could not find any option in SimpleMDE to disable autolinks. But I found that markdown2 can be configured to do it [3]. markdow does not have such extension but I guess it is doable to write one. For mistune, there is a url plugin [4].
I think we should activate by default this behavior (and maybe make it configurable).


[1] https://marked.js.org/using_advanced#options
[2] https://github.github.com/gfm/#autolinks
[3] https://github.com/trentm/python-markdown2/wiki/link-patterns#user-content-converting-links-into-links-automatically
[4] https://github.com/lepture/mistune/blob/master/mistune/plugins/extra.py#L15