I just want to add to this bug report an observation from my own experience.
i’ve tested this on Ubuntu 20.10 with Obsidian 0.12.5 and have the same experience that an external link within the page preview cannot be opened, however when I go one step deeper, i.e. I open a page preview of an internal link within the page preview of an internal link, the external link does work! Similarly, it works another level down as well, so I assume it is working correctly from there on out.
I dont see any relevant differences in the markup however. Both seem to have the same structure:
Debugging the javascript a little bit, I can’t find the problem though. Seems that both events (the functional and dysfunctional one) bubble correctly, etc.
Hope some of this detail helps you though.