I checked the recent changes to the parser code and this is likely the culprit: 0e1b52a4 "Make multiple colons escaping interlanguage links invalid, consistently" https://gerrit.wikimedia.org/r/#/c/361597/
Perhaps the change could be reverted, but given that (as you say) the problematic syntax is normally used in templates and not directly, perhaps the templates could be fixed instead.
Using "[[{{NAMESPACE}}:{{{1}}}|{{{1}}}]]" will not work correctly in the File and Category namespaces (it would generate a thumbnail or add the page to the category, respectively) – presumably the extra colon was to work around that. I would recommend the following:
[[{{#if:{{NAMESPACE}}|:{{NAMESPACE}}:}}{{{1}}}|{{{1}}}]]