The source message does not have that markup, apparently because it was meant to be plain text. The message was customized to use template on en.wiki to add underlining. So please ask them to either remove that template or completely delete the message so as to fallback to default.

Thanks for tracking down the edit that caused this problem. I contacted the editor who made it and he reversed it, and so the problem is fixed. Apparently the same text string is used in the “List of abbreviations” box in the upper right of the Watchlist and the markup does work there. The fact that the string is used in two places with possibly different results should at least be documented somewhere.

Best,

Arnold Reinhold

Thank you @ArnoldReinhold. Actually the text of the upper right box already has a separate message, MediaWiki:Wikibase-rc-wikibase-edit-legend but it was not defined. So the one for the tooltip was transcluded there. I will attempt the simple fix of defining it since both messages can be styled differently.