Searching the forum for the URL with the hypen yields results from 2020, so it’s not something that changed recently. Now whether there was a URL redirect in place in the past or not, I’ve no idea. Maybe? The only explanation I can think of.
The URL that you had bookmarked, appears on this forum for posts from 2020 and earlier. So I would guess some URL redirect was in place that has now been removed. I think after 3 years of removing the old URL, that it is perfectly understandable to do that since most links would have been updated by then to use the new one - including search engine results.
@iwalker Yes, you may be right that some redirect was in place.
I definitely know that this redirect was still working one week ago as in a certain environment I have a weekly script running checking URLs and it checked a lot of gitlab URLs one week ago successfully but now getting an error.
I noticed that the dash is required also for other repository URLs (perhaps for all, not sure).