Hello, Sam. Thank you for contacting us.
Indeed, the tab
Reports >
Sync Log is the tool you need for troubleshooting stuff like this. If you look at it more carefully, it should show exactly what happened.
When you select an Import or Export operation in the popup menu, it shows a list, giving the name of every changed item and indicating whether it was added, updated, deleted, or etc.
Going back to just after you added the new item, you should see an Import from Safari operation, and within its list that item should be listed as an addition. A minute or less later, you should see Export operations to other browsers, and within their lists, that new item should be listed as an addition. Typically the way new items disappear is that, before this export, for some reason which we need to determine, there is an Import operation from another browser which, of course, does not have this new item and consequently the new item is deleted. Or maybe the import from the other browser occurs before the import from Safari.
Either look at those Sync Logs more carefully and tell us what you see, or
send us a Trouble Zipper report, answering
Include Bookmarks and Documents when asked, and I can decipher it for you. Please tell me the name of at least one bookmark which disappeared, and the approximate time of day that you added it.