Its look like that within versions 170 & 172 markdown always open the link in a new tab, and there is no way to open the link on the same tab.
Any chance to roll back to the provisos mode? so you stay on same tab and click+ctrl or click on mouse wheel will open in new tab the link?
It doable ! If you want to open the link in a new tab use the full url, if you want to open the url in the same page just start the url from the # and on.
For more details check this thread