Pull up the pdf and open the signature panel. You won't find any digital signatures.
Yes, in MFB (I haven't looked at FF), you will find a graphic that indicates the original electronic record was digitally signed. That's just a graphical representation that indicates to the viewer that the original electronic record (the electronic logbook) was digitally signed. It is itself not a digital signature.
That's a very subtle but super important point, so I want to emphasize it: the signature (digital or wet ink) ONLY exists in its original form.
If it was ink on dead tree and you lose that logbook, you've lost the signature. If you had a photocopy of that, then you have proof of the existence of the signature, but it's not the signature itself. (Fortunately, that's generally good enough, or you can have a subsequent attestation of some sort that affirms that the original signature was valid, but again, that's not the original signature.)
It's similar in the electronic world: if an instructor signs a flight on electronic logbook X, then only X can guarantee that the signature is in fact valid. So if you export from X to a new electronic logbook Y, then Y CAN NOT validate that the original signature was valid. The original signature is still on X; Y can only record the existence of the signature.
To be a little more concrete: if you have a signed flight in ForeFlight, nobody other than ForeFlight can validate that the signature is true and correct, that it wasn't tampered with, that it was done by the CFI who it purports actually signed it. So if you export it to MyFlightbook, MyFlightbook could (in theory) prove that ForeFlight says the signature is valid***, but it would have to take ForeFlight's word for that (and, for that matter, ForeFlight has no incentive to provide such a validation service). Ditto if you export from MyFlightbook to ForeFlight.
So it's the subtle distinction between "this is a valid signature" and "this is a valid representation of a signature that was done somewhere else" - you're can validate the representation, but you can't necessarily validate the underlying signature.
This is a lot of angels-on-the-head-of-a-pin, and as a practical matter, I've yet to ever hear someone have an issue if they lose a logbook. But it is an argument for keeping original records.
E.g., I have PDF scans of all of my old primary training (paper) logbooks. In MyFlightbook, when I imported those flights, I put "[Signed by John Doe, CFI #12345 Expiration 10/31/2002]" or similar into the comments for the flight (i.e., REFERENCING the signature), and I have the PDFs themselves uploaded to MyFlightbook (under endorsements), so that if someone asks to see the original signature from John Doe, I can do that. And that's always been more than good enough. But neither of those is the actual signature - the actual signature is sitting in (now dry) ink on dead trees on a bookshelf in my house.
For years, I've recommended that people do something along these lines. There are comfortably into 6-figures of MyFlightbook users, and I've never had anybody have an issue around this...
***Cryptographically, you could imagine this being done with ForeFlight exporting a copy of the signed flight that is signed with Foreflight's private key; any recipient - with a public key that corresponds to that public key - could decrypt it and prove therefore that it was in fact encrypted by ForeFlight, and if you trust ForeFlight you could trust that the signature is probably what it purports to be. But that's proving that it came from ForeFlight, not proving that it's true and correct.