Please refer to this: https://help.salesforce.com/articleView?id=actions_in_lex.htm&type=0

In Lightning Experience, actions display in the Global Actions menu in the header, on related lists, on list view items, and in several places on a record page. Where they appear on a record page depends on the action’s type.

 

[...] The page-level action menu in the record’s highlights panel (1) contains: [...] Global and object-specific quick actions, except for those related to creating tasks, creating events, and logging calls. [...]

 

The Activity tab (2) contains Create a Record quick actions that point to the Event and Task objects. It also contains Log A Call actions and Send Email actions.

It's a built-in Lightning page behaviour. If you have a Quick Action that creates an Activity or sends an E-mail, it winds up in that tab instead of the button area. They just don't give you any choice in the matter.

Please refer to this: https://help.salesforce.com/articleView?id=actions_in_lex.htm&type=0

In Lightning Experience, actions display in the Global Actions menu in the header, on related lists, on list view items, and in several places on a record page. Where they appear on a record page depends on the action’s type.

[...] The page-level action menu in the record’s highlights panel (1) contains: [...] Global and object-specific quick actions, except for those related to creating tasks, creating events, and logging calls. [...]

The Activity tab (2) contains Create a Record quick actions that point to the Event and Task objects. It also contains Log A Call actions and Send Email actions.

It's a built-in Lightning page behaviour. If you have a Quick Action that creates an Activity or sends an E-mail, it winds up in that tab instead of the button area. They just don't give you any choice in the matter.