Steps to replicate

Initial setup

1. Login as admin.
2. Navigate to Site administration / Courses / Manage courses and categories.
3. Create a category called Drinks with the Parent category is Top.
4. Create a category called Coffee with the Parent category is Drinks.
5. Create a category called Italian coffee with the Parent category is Coffee.
6. Create a course called Espresso inside the Italian coffee category.
7. Navigate to that course.
8. Click on Participants.
9. Unenroll the admin user.
10. Create an assignment activity called "Test assignment".
11. Navigate to the Espresso course homepage.

Verify

1. Observe the top of the course homepage (1).
2. Click the Test assignment.
3. Observe the top of the activity page (2).{}

(1)

 - Expected result: There is no breadcrumb on this page.

 - Actual result: There is a breadcrumb on this page

(2)

 - Expected result: There is a short breadcrumb on this page with format [course_name] / [Activity name].

 - Actual result: There is a full breadcrumb on this page

— Original steps to replicate---

In https://github.com/moodle-an-hochschulen/moodle-theme_boost_union/pull/285#pullrequestreview-1415802009, abias describes an inconsistent behaviour inside Boost which you can reproduce.

FULL STEPS TO REPRODUCE
In an instance, for example https://school.moodledemo.net, as a manager do create a course category "Category EDCBA" as sub-category of a course category "Category EDCB" as sub-category of a course category "Category EDC" as sub-category of a course category "Category ED" as sub-category of a course category "Category E" on top level, then create five courses, one in each category.
Access those courses to compare, specifically the course in "Category EDCBA".

WHAT YOU EXPECTED
Either all courses feature the course category breadcrumb navigation, or none.

WHAT ACTUALLY HAPPENS
Beginning from the course in Course Category EDC (all hierarchically more nested), there is a course category breadcrumb navigation, up to there (all hierarchically less nested) there is none.

In https://github.com/moodle-an-hochschulen/moodle-theme_boost_union/pull/285#pullrequestreview-1415802009, abias describes an inconsistent behaviour inside Boost which you can reproduce.

FULL STEPS TO REPRODUCE
In an instance, for example https://school.moodledemo.net, as a manager do create a course category "Category EDCBA" as sub-category of a course category "Category EDCB" as sub-category of a course category "Category EDC" as sub-category of a course category "Category ED" as sub-category of a course category "Category E" on top level, then create five courses, one in each category.
Access those courses to compare, specifically the course in "Category EDCBA".

WHAT YOU EXPECTED
Either all courses feature the course category breadcrumb navigation, or none.

WHAT ACTUALLY HAPPENS
Beginning from the course in Course Category EDC (all hierarchically more nested), there is a course category breadcrumb navigation, up to there (all hierarchically less nested) there is none.