See these troubleshooting steps "Unable to see global or quick actions" Knowledge Article Number 000340216
Does your object use record types and is the testing user able to access them? See “Default Global Actions: A Special Case” help.salesforce.com/articleView?id=actions_and_record_types.htm&type=5
If so, then as of april 27, 2020:
Default Global Actions: A Special Case
If you have default global actions in your organization and you’re
using record types, your users might not be able to see all the
default actions that are assigned to a page layout.
Default global actions are assigned to the Master record type, which
isn’t accessible to most profiles. As a result, default global actions
with the Master record type that are associated with target objects
that have record types configured aren’t available for most users. To
fix this issue, edit the default global actions associated with those
objects and reassign them to a different record type.
For example, the New Contact default global action has Contact as its
target object. Let’s say you have record types set up for the Contact
object, and you add the New Contact default global object to a page
layout. Users who visit records based on that page layout don’t see
the New Contact action because the action is assigned to the Master
record type by default. Editing the New Contact default global action
and assigning it to a record type other than Master makes it available
for all users who have access to its assigned record type.