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In 4.20 the way the UI system treats connections with the UIui-tag was changed in such a way that as of 4.20 default UI - connections are required to only have a single tag (the ui-tag). Before that change, default UI - connections could have other tags in addition to the ui-tag.
The problem with the old behavior was that in certain asset set ups, the UI would select the wrong UI- connection onnection (mistakenly taking a non-default ui-tag for a default one). While the assets in a vanilla game are supposed to not suffer these problems (aka: where the game used a default - ui - connection, the chosen ui- connection in the end practice always was always the default one), it's possible for mods to have created/changed assets resulting in the wrong connection being used.
The actual player visible/notable effects are quite difficult to describe in detail, because they depend on several game states which are not linear to reproduce. Notable problems ranged from wrong or duplicated target elements having been displayed, cases where the wrong target was selected or others were not selectable , other targets not being selected at all, or up to less perceptible issues like timings where a target element changed were slightly to change the current target being off.
We don't expect that any mod out there actually ran into this problem though, but if you did and you expected the previous old behavior, feel free to leave a comment here.
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