Lap markers are missing from the maps in old activity records, even though there were clearly splits in the track.
The oldest activity record of mine for which I can see lap markers on the map was from 18 August 2016. However, I didn't record any timed activities for over a month before that, so I cannot ascertain whether the issue is only affecting activity records from before a particular date, e.g. 31 July 2016, or only affecting activity records more than a year old (as of today).
Now, interestingly, when I exported a thus-affected activity record to a TCX file, then replaced the timestamps globally in the file to put the activity on a different date, then imported the modified file back into Garmin Connect as a different activity, I can see the lap markers in the map for it. It does not matter whether I put the date of the activity record forward or backward; I have already tried both. So, the criterion in question would appear to have something to do with the sync or upload date for an activity record, and not the activity date itself.
