Issue originally reported by user jccemden at Joomla-Bugs.de. Confirmed by me.

Steps to reproduce the issue

1. Create an article "Archived January 2018"
   • Creation date: January 2018 (day doesn't matter)
   • Status: Archived
2. Create an article "Archived January 2017"
   • Creation date: January 2017 (day doesn't matter)
   • Status: Archived
3. Create an article "Archived February 2017"
   • Creation date: February 2017 (day doesn't matter)
   • Status: Archived
4. Create a module of the type "Archived articles" and publish it.
5. Enable SEF
6. In frontend, the module should display the months for which there are archived articles:
   • January 2018
   • February 2017
   • January 2017
7. Click on "January 2017"

Expected result

You should only see the article from January 2017.

Actual result

Instead, you'll see the "January" articles from all years. The month filtering works, as you don't see the "February" article, but the year filtering does not.
It works if SEF URLs are disabled. However, there is an additional issue (not sure if related or not) in both versions: The filter values are not or only partly propagated to the filter form. If you click on "January 2017", I'd expect the filter form to hold "January" as the month value and "2017" as the year. For SEF URLs, both are empty, for non-SEF, the year is entered, but the month is missing.

System information (as much as possible)

The issue exists in Joomla! 3.8.3 and in the current staging. Tested on PHP 7.1.9.

Additional comments

I'm not sure if this issue is related to the already known routing issues in 3.8. Due to limited time and a broken development environment, I'm not able to test the proposed PR(s) right now to see if they fix this issue, too.