The issue still occurs, any update?

I'm having the same issue.

@Eonasdan Could this issue be simply fixed by changing line 1151?
Change from
TempusDominusBootstrap4._jQueryInterface.call($target, config, event);

Change to
`TempusDominusBootstrap4._jQueryInterface.call($target, 'show', event);

Just tested: yes, that fixes it. Just created an PR for it: #106

Not sure what happened since I made the PR and now it's been merged, but apparently the actual field now also needs to have the data-target set (after initialization).

I think it can be solved by checking if the element has the datatimepicker data object available:

if ($element.data(DateTimePicker.DATA_KEY)) {
 $selector = $element;
} else {
  return $selector;
}

at line 19 in the JS.

@Eonasdan Can you confirm?

Issue still exists.

I've created a PR with my solution, as that works for me. If you're still waiting for this, you can use this workaround:

// Custom focus/blur handlers
$(document).on('focus.datetimepicker', '.datetimepicker-input', function () {
    $(this).datetimepicker('show', $(this).data('datetimepicker'));
});
$(document).on('blur.datetimepicker', '.datetimepicker-input', function () {
    $(this).datetimepicker('hide', $(this).data('datetimepicker'));
});