Hi Walter,
This is not an issue its the default functionality and how it you should behave.
That is because when you don’t set the hours and just the date like in your case (to either dateStart or dateEnd) the date that the gantt chart will output is the following (startDate : Fri Apr 02 2021 00:00:00 , endDate : Mon Apr 05 2021 00:00:00) and thus the cells that the task is covering start for the beginning of the cell on 02.04 date and ends on the very end of 04.04 date cell.
However if you want the task to cover the cell you will have to set hours,minutes and seconds.
For example in order the cell on 05.04 date to be covered you can set the dates like this :

    label: 'Interior Arrangement',
    dateStart: '2021-04-02 16:30:59 ',
    dateEnd:'2021-04-05 23:59:59',
    type: 'task',

Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team