I have that exact same problem too. Matter of fact, I even have a
video on my YouTube channel that shows what's happening in the "My Designs" tab. I was able to get FH4 for my 30th Birthday in early October 2020, less than a month before this bug appeared. I was able to import some of my FH3 designs, and now I can't import the rest of them now because of this bug!! Seriously, if I knew that this was gonna happen, I would've imported all my FH3 designs on the day I started playing!! All I can say is that this is a bug that resulted from the Series 29 Update. Matter of fact, I have been wondering if there's a way to roll roll the game back to the Series 28 Update or earlier, as this might re-enable me to import the rest of my FH3 designs.
NOTE: At the time of the video, my Xbox gamertag was "AgentRed1927", but in early January 2021, I changed it to "Ytbr77Pacer".
Edited by user Thursday, February 11, 2021 10:40:10 AM(UTC)
| Reason: Not specified