This is more a usability issue than a 'bug' per-se. I would recommend you include the below CSS enhancement in a future release.

Issue description:

In the new version, the select filter dropdown lists have very little indication that they are select boxes. The old version had the box/outline around the field, and also had placeholder text. The new just has an unobtrusive light grey carat on the right.

The "unobtrusive" appearance is awesome for regular users, but we have periodic users which could use more obvious visual cues.

My table configuration includes both: "Enable advanced column Filters" and "Show filters in a form above the table".

I have included the text placeholder "[any]" via the following CSS in my theme overrides:

/* put some text in the null filter select-box option */
a.search_init::after,
.wpdt-c .bootstrap-select > button.bs-placeholder .filter-option::after {
    content: "[Any]";
    color: lightgray;
}

Thanks!
Corky.

P.S. - I have a couple other issues I'll submit separately for simpler tracking.