The Forza team has a list of Known Issues they are working on. Until all are resolved, some players will be affected and frustrated not only by the bug but because the team cannot provide a timeline for when everything will be resolved.
There is a limit to the number of saved files in the game which is lower than the total number of cars available to own. Again, this is not a desirable feature, but an in-game limit is not necessarily a bug. We encourage you to vote on topics where you’d like to see game features introduced or changed, such as the file save limit which we can use to encourage the developers to look at.
For topics that are listed as Bug-Logged in the Known Issues section of the Troubleshooting Hub, the situation is as was described to you above - the team is investigating and at some point may be able to address it, but until then we have no further information.
Please read through our Suggestions Hub and Troubleshooting Hub articles to understand the process.
Until the topic is addressed, the workaround is to delete files when you reach the limit, or if deleting and the steps provided by Support do not clear an error, wait until a fix is deployed. If it’s not fixed yet, it’s not fixed. “We will send out an announcement to the Community when we have a resolution.” Once this has been stated by Support and a ticket has been closed, please do not try to reopen the same topic. Thanks for your patience.