The 1999 date issue is fixable by exporting the gpx and re-importing after making some changes
Like many, I've been experiencing the "1999 date issue". See https://support.strava.com/hc/en-us/articles/360026660091-Activity-Syncing-Issues-and-Incorrect-Start-Times-beginning-on-April-6th- for an official Strava update on this.
In that post they mention they cannot fix the 1999 date. That's unfortunate if you want your stats to be accurately reflected. However, you can actually fix it as follows:
1. export the activity as a .gpx
2. open the file in a text editor
3. replace all occurences of the wrong 1999 date with the date on which you actually recorded the activity
4. (optionally also change the time although that's much more troublesome)
5. upload the changed gpx file
6. verify it now appears in your feed where you expect it to
7. delete the original
Hope that helps anyone.