We don't know if this is what is causing the issue for everyone else, but:

We have discovered that the new add inventory process does not take into consideration previously used SKUs. So, if you use an old SKU, it will be allowed. The previous version recognized previously used SKUs, and did not allow the item to be listed using the same SKU. This was before amazon made their improvement on the system.

If you use an old SKU, it will list the item but simultaneously remove the old listing that had the same SKU. If this happens, it will cause additional listings to fail to show in inventory. If we subsequently search for the items that were not listed, the search results will often result in a different title (that was also recently listed) that does not match the search terms. Multiple missing items can show up under the same erroneous listing in the search results.

For example, if we list a Harry Potter book with a previously used SKU, it will kick out the old SKU (even if it is years old - the timeframe does not matter). The next book we list, Twilight, will likely be successful, but from this point on, we will have random other new listings disappear. If we search for a disappeared title after allowing enough time for everything to populate the inventory, then it will bring up Twilight, the title that we immediately listed after Harry Potter (Potter being the SKU that was duplicated). Nothing after the point at which the SKU was duplicated can be considered accurate. Once this happens, we have to stop listing and wait for an entire day, since waiting for several hours will not suffice in resolving the issue.

We hope @Manny_Amazon might be able to communicate this to the correct department, since it is something complicated enough that it cannot be successfully communicated to seller support.