Leonin Arbiter behaves weirdly on Magic Online: if you want to pay 2 to search your library you must click the arbiter before you search

[datrobino]

Please provide a list of affected cards Affects: [Leonin Arbiter]

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[emerald000]

That's how it works according to the rules. You need to pay the {2} as a special action before you search your library.

[stash86]

It's the same as if you want to activate an ability of your creature, you need to click on it. So the way as arbiter. You can pay 2 at any time you want by clicking on it. It's not a triggered ability, so don't expect to get a prompt by the client