Issues with WP 5.5.0

When loading the page editor, the following error is briefly visible, though hidden once the page editor loads:

Notice: register_rest_route was called incorrectly. The REST API route definition for oxilabtabsultimate/v1/(?P\w+) is missing the required permission_callback argument. For REST API routes that are intended to be public, use __return_true as the permission callback. Please see Debugging in WordPress for more information. (This message was added in version 5.5.0.) in /nas/content/live/[site-root]/wp-includes/functions.php on line 5311

It also appears to mess with how select boxes are rendered in the Settings panel along the right side of the page/post editor. Instead of properly being rendered with a bordered box, the enclosing box is collapsed to a single line.

Both of these issues only appear when this plugin is active (and when it’s the only plugin active), and disappear when it is not. It also doesn’t appear to be an interaction with any specific theme.