When I select the time first in the date time field it does not hold the value and when I do the same again and select the time first it holds it. Below is my code : 

JS : 
 

    @api endDateTime = null;
    handleEndChange (event) {
        this.endDateTime = event.detail.value;       
    } 
               
html : 


    <lightning-input class="" type="datetime" name="End" label="End" value={endDateTime} onchange={handleEndChange} message-when-value-missing=" " message-when-bad-input=" " required></lightning-input>

[![enter image description here][1]][1]

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/zKUBE.png
  [2]: https://i.sstatic.net/rF0hs.png