If you can't display your ticket then you have committed an offence. It doesn't matter whether this is because your phone glitched and refused to display an e-ticket or because you lost a paper ticket. It is your responsibility to ensure you can show your ticket.
I can understand why you are frustrated by this, but those are the facts.
Penalty Fare appeals are conducted in three stages. Stages one and two are conducted by 'independent' appeals companies which are, truthfully, anything but independent. Of the two appeals companies, one makes most of its money selling revenue protection software to TOCs and the other was actually directly owned by a TOC until fairly recently (and still has the same management as when it was owned by that TOC).
It is only the third appeal stage that is truly independent. As
@AlterEgo says, unless you show the Penalty Fare notice to us and we can identify any issue, any appeal will be on the grounds of 'compelling reasons'. Buying a ticket and having a technical issue
may be accepted as a compelling reason, but there is no guarantee- it is at the discretion of the appeals panel. If you can provide evidence of the ticket then they are certainly more likely to consider your 'compelling reasons' than either of the first two appeal stages are, but it is not certain. You may also wish to explain any other mitigating circumstances, such as whether you were given sufficient time to try and rectify the issue, e.g. by giving you sufficient time to attempt a fix such as rebooting your phone, and whether you were subsequently able to fix the issue.
I would appeal though, right to the end, make them earn their £50.