These controls do not take the width of the input group, like standard controls do.

Confirmed for the following controls, but may be true for other form controls too:

• c-date-picker
• c-time-picker
• c-multi-select
• cFormPassword

I can fix this by adding the following block to my custom stylesheet, which i copied from the standard input-control:

.form-password, c-date-picker, c-time-picker, c-multi-select {
  position: relative;
  flex: 1 1 auto;
  width: 1%;
  min-width: 0;
}

But i think, CoreUI stylesheet should handle that.