I have overridden standard 'Add Products' button of OpportunityLineItem object with a VF page. It was working fine in classic and Lightning both before Spring'18 release. But after Spring'18 release it is working only on classic. In lightning it is redirecting to the standard detail page of opportunity. Is this any issue with Spring'18 release or am I missing some settings for button override.
After Spring' 18 you need to assign separate standard action overrides for Different Experiences.
The action override screen for standard actions, like New, View, and Edit, has gotten a makeover. Now you can easily assign different overrides for Salesforce Classic, Lightning Experience, and mobile. For example, for the same standard action, you can set a Visualforce page override in Salesforce Classic, a Lightning page override in Lightning Experience, and a Lightning component override in mobile. This change applies to Lightning Experience, Salesforce Classic, and all versions of the Salesforce app.
1st button with Name AddProduct is configured on Salesforce classic. and the other button with Name MultiAdd is configured on Lightning experience.
So as per my understanding, you need to override the VF page on second button as well. Please verify and let me know if that works for you.