In Lightning Experience, the "Feed View" set on the page layout is not applied.
This feature is only available in Salesforce Classic.
Use Quick Actions instead in Lightning Experience.
In Lightning Experience, the "Feed View" on the page layout is not applied; instead, Quick Actions are used.
By creating a Quick Action and placing it on the page layout, you can use it from the [Feed] tab.
Quick Action Help:
Create Object-Specific Quick Actions
Page Layout Help:
The Enhanced Page Layout Editor
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