Possibly because you did the Activity already past midnight UTC (Coordinated Universal Time). Try editing the Activity, and pushing the time of the beginning a few hours back. Not sure whether it can work, but it is worth of trying. If it does not, another possibility would be exporting the Activity as "Original" through the cogwheel menu in Garmin Connect Web, modifying the time with the time Adjuster of FIT File Tools, and then importing back to Garmin Connect.
I adjusted my last run (June 30) to earlier in the day.
Interestingly enough, after I adjusted the time, I noticed that the date said, July 1??? I changed it to June 30 (the original running date) and suddenly all of my miles reappeared. Even though I ran 100.5 miles, the app says I ran 99.
