When the material form field appearance is set to outline, and the field has no value, the floating label blocks the click action from opening the dropdown. I've confirmed this is only an issue for outline, the default (fine) is fine.
Here is a stackblitz to reproduce the issue: https://stackblitz.com/edit/pqp6chgz?file=src%2Fexample%2Finput-overview-example.html
Here is an image showing the click region of the label:
