I believe NNGroup has one article mentioning some different scenarios for placeholder text in form fields. What's good is that the content from NNGroup is also based on user research and usability testing. One caveat though is that the research seem to mainly mention text fields and not dropdowns, so I am a bit unsure on how well the research will apply to your scenario. This is the closest kind of research I've managed to find about this. To be 100% sure I do recommend to perform some very light-weight usability testing and see if your design works or not.
Here are 7 reasons, taken from the article above, why placeholders as labels should not be used when replacing field labels:
- Disappearing placeholder text strains users’ short-term memory.
- Without labels, users cannot check their work before submitting a form.
- When error messages occur, people don’t know how to fix the problem.
- Placeholder text that disappears when the cursor is placed in a form field is irritating for users navigating with the keyboard.
- Fields with stuff in them are less noticeable.
- Users may mistake a placeholder for data that was automatically filled in.
- Occasionally users have to delete placeholder text manually.
Here is what the same article has to say about your specific example, where placeholder text is used in addition to labels.
Placeholder Text in Addition to Labels
Using placeholder text in combination with form labels is a step in
the right direction. Labels outside the form fields make the essential
information visible at all times, while placeholder text inside form
fields is reserved for supplementary information. However, even when
using labels, placing important hints or instructions within a form
field can still cause the 7 issues mentioned above, albeit with less
severity. If some of the fields require an extra description that is
essential to completing the form correctly, it’s best to place that
text outside the field so that it is always visible.
One solution to this problem could instead be to have text in close proximity to the field: