D365 CE UCI : Custom Button in command bar does not appear after grid item selection

Introduction

Recently, I have noticed the weird behavior on UCI Sub Grid Button. It was working properly on legacy UI but disappeared in new UCI app whenever I select any record .

While browsing, I have found some article which was related to this but it was a temporary solution.

Resolution

1. Open the Ribbon work bench on Xrm Toolbox and select particular entity.
2. Add new Enabled rules with below settings
   Id : Mscrm.AnySelection
   Is Core : True

3. Associate this enabled rule with Button Command and publish your changes.

Button will appear in new UCI after the record selection.
Cheers

https://support.microsoft.com/en-in/help/4481268/button-in-command-bar-does-not-appear-after-grid-item-selection