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}}}]]
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Bartosz Dziewoński