New Button Greyed Out on Initial Page Load

Our client has reported a rather odd behavior involving list view web parts on a page on a classic page after installing the May CU. The “(+) New” button is greyed out on the initial load of the page and is not clickable. But if you click anywhere inside the webpart, it lights up and turns blue. Every time the page is reloaded or revisited, the New button reverts to grey and unclickable. The source list works fine in both classic and new experience.

This is a new behavior change. Has anyone else observed this?