Steps to replicate

1. Login as admin.
2. Open the user profile menu.
3. Click on Calendar.
4. Click on the Import or export calendars link.
5. Click on the Import calendar button.
6. Set the Calendar name to: Test ICS.
7. Set the Import from to: Calendar file.
8. Select the Timetable-Lecture.ics file.
9. Press the Import calendar button.
10. Open the user profile menu.
11. Click on Calendar.
12. Navigate to May 2022.
13. Click on the 27th of May 2022 to view that day's detail.
14. Verify that you will see 2 events: Concurrency and Introduction to Formal Proof.
15. Observe the description of both events.

Expected result:

1. Description of Concurrency event is: Concurrency, A lecturer http://some.alternative.representation/
2. Description of Introduction to Formal Proof event is: Introduction to Formal Proof, A lecturer http://some.alternative.representation/

Actual result:

1. The Concurrency event does not have a description.

If an iCalendar .ics has a DESCRIPTION property and that has an ALTREP parameter, the description may not import and is not shown in the calendar/view.php

https://datatracker.ietf.org/doc/html/rfc5545#section-3.2.1

e.g. this imports:

DESCRIPTION:Introduction to Formal Proof\, A lecturer

this does not:

DESCRIPTION;ALTREP="http://some.alternative.representation/":Concurrency\, A lecturer

Looking at the regex in https://gitit.nmu.edu/clewis/moodle/-/blob/v2.8.0/lib/bennu/iCalendar_components.php#L246 and putting that into https://regex101.com/

it parses.

The expected behaviour is that both descriptions show.