Lap markers on mobile appear every 1km but only every 5km on desktop for same run????

I noticed the same symptom.
In my case, I don't know how GC Desktop decided to show or hide my split markers.
It doesn't seem to make any sense.

https://connect.garmin.com/modern/activity/1176187954

Interesting! When I opened GrandWaz00's activity, it only showed me every 10th split on the map - only 7 of them, starting with the 9th kilometer up to 79th.
I started thinking it might be the way autolap is set up on Garmin Connect (as opposed to on the device itself), but when I entered my Edge 25's device settings in Connect, it displayed no data (even though on the device itself it's set up to 1km) - and a blank drop-down menu. I suppose it's (yet another) bug on the site.

I can confirm (at least, for the Fenix F3) that the watch is recording the data.
Of course, if GC Mobile is showing the splits, then they were recorded properly.
I've downloaded and examined the FIT file, and all the splits are in there.
Also, I've downloaded the "Google Earth" version of the download, and loaded it into both Google Maps and also Google Earth, and can see all the split markers are present in both.
So, the issue seems to be limited to GC Desktop's rendering of the splits along the path.

I'd also like to note, just in case the question arises, that the path I submitted above was a record of a bicycle tour.
We were given turn-by-turn cue sheets, and I manually recorded a split at (almost) every turn.
No automatic split/autolap was used.

Hopefully someone familiar with GC Desktop will notice this thread and reply.

For activities that have ten laps or less, all lap icons will display on the map. For activities that have greater than ten laps, the number of icons will be reduced to prevent that map from becoming too cluttered with icons. Garmin Connect will display a maximum of ten number icons in an activity.

The reason why you are seeing all the splits when viewing the map on your mobile browser is, the mobile web browsers is pulling/displaying all the information from the server. The Connect team has not coded Garmin Connect to display the same way in mobile browser views because most customers are not viewing Garmin Connect in a mobile browser. Users typically are viewing Garmin Connect on a desktop or using the Garmin Connect Mobile app.

If I may say so, I actually prefer the GCM display of "all splits shown"

This works well for me, because I use a "split" to mark a "point of interest" (a turn) along a linear path, and NOT (as others may do) to indicate a LAP (a specific location that is crossed multiple times during a workout).

On GC Desktop, if I don't like the clutter for some reason, then I can always uncheck the "show splits" checkbox

If I choose to view the splits (my bike tour cue sheet, for example, had turns as frequently as every 0.1 km), then I can simply zoom in.

It might look like clutter from 50,000 feet, but it is data that I chose to record, so I'd like to see it on the map :-)

The reason I'm interested in the splits (which I tried to take (mostly) great care to mark, using my watch) is so I can review the route with the bike tour leaders, and show them where, for example, I misinterpreted their written instructions and made wrong turns.

So I'm very happy that GCM displays them all (because I don't bring my laptop on bike tours), but when I email the GC activity URL to the tour leaders, most of the splits are missing on the GC Desktop map, so it loses its usefuless

As I said, I have a workaround (by exporting the "Google Earth" file, and loading it into Google Maps), but this must wait until after I return home from my trip, and is rather time-consuming, so I'd probably prefer not to do it if I don't have to

Thank you for your reply, and for considering this point of view.

We always appreciate user feedback, and any suggestions that you may like to see for changes to Garmin Connect can be submitted to www.garmin.com/ideas. This information is provided directly to the developers and all suggestions/ideas are evaluated and could potentially be used in future updates.