Reported at the Support Desk.
I've tested this on current master, on 1.28, 1.27 and 1.25 with the same results. However, I'm unable to reproduce this on WMF wikis.
Minimal test case: Try to parse this:
<abbr>[[x]]</abbr> [[y]]
(convenient URL fragment for Special:ApiSandbox): /Special:ApiSandbox#action=parse&format=json&text=<abbr>[[x]]<%2Fabbr>+[[y]]&prop=text|parsetree&disablelimitreport=1
This results in:
{ "warnings": { "parse": { "*": "No \"title\" or \"contentmodel\" was given, assuming wikitext." } }, "parse": { "title": "API", "pageid": 0, "text": { "*": "<p><abbr></a><a href=\"/index.php?title=X&action=edit&redlink=1\" class=\"new\" title=\"X (la p\u00e1gina no existe)\">x</a><abbr></abbr> <a href=\"/index.php?title=Y&action=edit&redlink=1\" class=\"new\" title=\"Y (la p\u00e1gina no existe)\">y</a>\n</p>" }, "parsetree": { "*": "<root><abbr>[[x]]</abbr> [[y]]</root>" } } }
As you can see, after the opening <abbr>, there's a closing </a> (!) and after the second closing </a> there's an empty <abbr></abbr>
Issue not reproducible with only one link.
There seems to be a similar issue reported on T24905 but fixed long ago.