In 1.13.16, the title attribute gave a placeholder when no option is selected.
This functionality does not work in 1.13.17
<select class="selectpicker form-control" id="number" title="Select a number"></select>
This should show Select a number when no option is selected.
Test case: https://plnkr.co/edit/eStCUygC40HAM52x
This is working for me in Chrome 81 and Firefox 76 on Windows. What browser/OS are you using?

I'm getting this bug on MacOS, Safari 13.1.

Interestingly, it works fine on my Chrome in MacOS.
Released in v1.13.18!