As for M5 the mutisite module is the successor of the ETK module, I created this issue.
It is a "clone" of MGNLETK-111.

Same content/description is valid:
"If you are using a variation on site definition like the smartphone variation, all links on the smartphone view are rendered as external (complete with host) links.

The reason for this bug is, that the SiteMergeFilter creates site proxies, if a variation is used. The equals comparison in the CompoundLinkTransformer fails on usage with site proxies. And so the link transformer creates external link for links in same site, too.
(Specifically isTargetExternal returns true, so 'result = linkManager.getCompleteUrl().transform(link);' is the result.)

To reproduce this issue create a new home page and view the source code of the smartphone preview iframe."

Note that this is only only occurs with pages in sites that have no domain configured, such as the stock default site.