fullcalender last day of week has wrong width

Question

I am using FullCalendar v1.6.4 and I have problems with events that last for several days and that go over Sunday. The event on Sunday always has the wrong width. In my example I add a event like this:

var eventObject = {
                    title: event_text,
                    start: f,
                    end: u,
                    id: 1,
                    color: bgcolor,
                    textColor: "#fff"
                    };

Start is 2021/03/21 and end is 2021/03/22. So two days. Sonnday until Monday.

The event on Sonday gets a width of 11px instead of 156px like all other events:

<div class="fc-event fc-event-hori fc-event-start" style="position: absolute; left: 951px; background-color: rgb(0, 204, 0); border-color: rgb(0, 204, 0); color: rgb(255, 255, 255); width: 11px; top: 334px;"><div class="fc-event-inner"><span class="fc-event-title">NEW EVENT</span></div></div>

Bests, Yanick