Transfer Copy Filed no longer there

Good morning

We are doing an application for the registered proprietors of the attached title. On our recent title, it refers to a Transfer dated 24th April 2007 at C2, with 'NOTE: No copy of the Transfer referred to is held by HM Land Registry' underneath.

However, when asking our clients about this, they advised that on the title they received when they purchased the property, the same entry says 'Copy Filed' underneath (see attached screenshot of the same).

Is there any reason why this transfer would no longer be available after initially being filed?

Thank you

Good Morning Mark - we update the register to state no copy filed if someone applies for a copy and we discover that the copy as originally filed cannot be located/scanned for example 
This is what happened in 2021 and the register was updated accordingly 
If your clients have a certified/official copy or the original then you can submit same and ask us to update our records as appropriate
HD468476