I'll reword my initial post to say that screenshots of tickets are valid, if the scanner accepts them, however there is more potential for problems and rejection of the ticket compared with pdf files or printed versions, because of problems reading the screen. That's why a lot of TOCs only mention pdf or print out and don't recommend using a screenshot.
If a passenger prints numerous copies of the same ticket to give to accompanying passengers, the scanners at my TOC's stations will accept one copy but then reject the others, to prevent fraud. Providing no fault with the scanner, it should have no problem reading a valid ticket in paper or pdf form.
However there is a grey area if a seemingly valid e-ticket is displayed on the phone in screenshot-form and the scanner rejects, as it could either be because of fraudulent use with the passenger having been sent a screenshot or taken a photo of someone else's ticket, or because there is a problem with the device screen, or a blurred photo. Revenue staff have to make a decision in this scenario and if they suspect fraudulent use, they are allowed to refuse the ticket.
This is why TOCs don't like to promote screenshots of e-tickets & in some cases state they're invalid, and by promoting them here people are undermining the TOCs.