Creating a outlook calendar event leaves a shade of that event throughout the entire day till the end date

Question

Alright so if I would create a event in my calendar manually then if I would add an all day event then I would get just a bar in the top of my dat with the event data in it.

But if I would try the same movement with my code and the api then it would add a shade of that event category color throughout my entire day (added a picture as attachment)

The strange thing is, yesterdat when I used my code to update the event it removed the shade effect but today its no longer doing such

I have also attached an image where it shows the timing

What could be wrong? if its an all day event then it shouldn't add the shades but there's no option in the api to disable it, It might be because I've got the option within my application to select multiple days but then I still don't get the shade part.

I've tried changing timezones, tried to pass in 23:59:59 as end time in stead of 00:00:00, tried to have no all day status but instead setting the start time at 00:00:00 and the end time at 00:00:00 at the next day, that didn't work either

Contacted our IT provider but they weren't familiar with this issue

'start' => [
    'dateTime' => $startTime->startOfDay()->toIso8601String(),
    'timeZone' => 'UTC',
],
'end' => [
    'dateTime' => $endTime->startOfDay()->addDay()->toIso8601String(),
    'timeZone' => 'UTC',
],
'isAllDay' => true,

Calendar overview with the shades