General journal lines are deleted after posting
Unfortunately, your screenshot doesn't address my question. That you don't see something in a form doesn't mean that it doesn't exist. The records may exist, they're just not shown because they're not returned by the query currently used by the form. For example, there may be a filter added in code or by a user.
In short, just looking at the form doesn't tell you that the records were deleted. That's just one of the options. You should test your assumption before you invest a lot of time on investigated based on this - possible incorrect - assumption.
You could use the table browser to verify it. And if it's a non-production environment, you can look directly to database.