This is an older ticket but I ran into this same issue and found a work around that might work for you. I took idea from the post listed in the Admin notes. (https://feedback.telerik.com/blazor/1502395-ability-to-directly-edit-an-occurence-or-the-series-without-the-prompt-asking-you-to-choose) and changed it to work off of the default Cancel handler.

So what happens is the window will prompt to edit occurrence or series, then when the user makes a selection, the Edit Handler will capture the default meeting info (this seems to always be the series), then the default edit window will popup for a split second and a javascript function will automatically hit the cancel button. Then the cancel handler event args actually grab the correct start/end times. I compare that appointment to the default one. If the default appointment had a recurrence but the cancelled appointment didn't, then I know someone was trying to edit an occurrence and create an exception. I build the correct appointment object with the start/end and recurrenceId fields as needed and then open my custom editor with that info.

Example:

<script suppress-error="BL9992">
    function clickSchedulerPromptButton(btnIndex) {
        setTimeout(function () {
            var buttons = document.querySelectorAll(".k-dialog-content .k-form-buttons button");
            if (buttons && buttons.length >= btnIndex) {
                var chosenButton = buttons[btnIndex];
                chosenButton.click();
            }
        }, 100);
    }
</script>

public async void EditHandler(SchedulerEditEventArgs args)
        {
            var item = args.Item as MeetingViewModel;
            IsNew = args.IsNew;

            if (!args.IsNew) // an edit operation, otherwise - an insert operation
            {
                DefaultMeeting = item.ShallowCopy();
            }
            else
            {                
                DefaultMeeting = new MeetingViewModel() { Start = args.Start, End = args.End, IsAllDay = args.IsAllDay };
            }
            int btnIndexToClick = 1;

            await _jsInterop.InvokeVoidAsync("clickSchedulerPromptButton", btnIndexToClick);
        }

public async void CancelDefaultEdit(SchedulerCancelEventArgs args)
        {
            var item = args.Item as MeetingViewModel;
            if (!IsNew) // an edit operation, otherwise - an insert operation
            {
                CurrentMeeting = item.ShallowCopy();
            }
            else
            {
                CurrentMeeting = DefaultMeeting;
            }

           // If True then user was trying to create an exception. Every other instance will be False.

            if(!string.IsNullOrWhiteSpace(DefaultMeeting.Recurrence) && string.IsNullOrWhiteSpace(CurrentMeeting.Recurrence))
            {
                CurrentMeeting.MasterRecurrenceId = item.Id;
                CurrentMeeting.Recurrence = null;
                CurrentMeeting.RecurrenceExceptions?.Clear();
                CurrentMeeting.Id = 0;
            }
            CustomEditFormShown = true;
            await InvokeAsync(StateHasChanged);
        }