The issue must related to a change happening on 13 March or a few days before.

This is also happening whenever the auto-completion popup list is closed and reopened.. so effectively, it is working as expected only the first time the list appears, regardless of which item is added.

Side note – you don’t have to enter any grammatical features by hand if you use the Wikidata Lexeme Forms tool :) @Fnielsen perhaps you’d like to add some templates in Danish?

This seems to be a change in OOUI – you can experience the same behavior on the OOUI Demos page, in widgets like “MenuTagMultiselectWidget (allowArbitrary: false)”.

Hm, as far as I can tell from local testing, this regressed between 1.33.0-wmf.20 and wmf.21, so it would have broken on Wikidata around 2019-03-13.

…and I just realized that this is exactly what @Fnielsen wrote (for some reason I misread his comment as April 13 and was confused about the mismatch), so that makes perfect sense. Investigation continues…

git bisect in OOUI points at rGOJU2d4c8af00035: MenuTagMSW: Use 'highlightOnFilter' flag in MenuSelectWidget, so CCing author @Esanders and +2er @Mooeypoo for their information, since this might affect other OOUI users as well and I don’t know if the OOUI team wants to backport the fix somewhere or anything like that.

I’ll try to fix this tomorrow. In the meantime, here’s a short reproducer that can be used with an HTML file like in the OOUI ToDo App Tutorial:

$( function () {
    var widget = new OO.ui.MenuTagMultiselectWidget( {
        allowArbitrary: false,
        options: [
            { 'data': 'abc', 'label': 'Label for abc' },
            { 'data': 'asd', 'label': 'Label for asd' },
            { 'data': 'jkl', 'label': 'Label for jkl' },
        ],
    } );
    // Append to the wrapper                                                                                                                                                 
    $( '.wrapper' ).append( widget.$element );
} );

Change 504546 had a related patch set uploaded (by Lucas Werkmeister (WMDE); owner: Lucas Werkmeister (WMDE)):
[oojs/ui@master] MenuSelectWidget: Fix highlight on redisplay

https://gerrit.wikimedia.org/r/504546

Change 504546 merged by jenkins-bot:
[oojs/ui@master] MenuSelectWidget: Fix highlight on redisplay

https://gerrit.wikimedia.org/r/504546

Problem on the demo site fixed, this just needs an OOUI release now.

Change 506208 had a related patch set uploaded (by Jforrester; owner: Jforrester):
[mediawiki/core@master] Update OOUI to v0.31.5

https://gerrit.wikimedia.org/r/506208

Change 506208 merged by jenkins-bot:
[mediawiki/core@master] Update OOUI to v0.31.5

https://gerrit.wikimedia.org/r/506208

This seems to work fine now.