Before you start removing/updating Thunderbird, we first have to make sure you are *really* on beta. For me it looks like you are on Release, but the build uses a wrong name to identify itself. The screenshot I shared shows that the update-channel is "release", even though the title says otherwise. Beta by the way is on 139.x. It can very well by that the bug regarding calendar is indeed a bug in Release.
Bug 1964734 Comment 6 Edit History
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Before you start removing/updating Thunderbird, we first have to make sure you are *really* on beta. For me it looks like you are on Release, but the build uses a wrong name to identify itself. The screenshot I shared shows that the update-channel is "release", even though the title says otherwise. Beta by the way is on 139.x. It can very well be, that the bug regarding calendar is indeed a bug in Release.