mobile phone cameras often don't have enough resolution for the amount of data in a lightning invoice. if the QR is only an LNURL like LUD16 you get less problems.
Discussion
Switching to a different app on the same phone often worked though. So we swapped to ZEUS or WoS after Phoenix failed and those apps could often fetch the invoice 🤷♂️
Maybe Phoenix was just bad 😆
probably depends on the QR code library in use then also
oh yeah, most phones have a good scanner in the phone now. you can scan with the camera, copy, and paste into the lightning wallet and pay that way for this case. but try other LN wallets too, and if you love the one you have, bitch at the devs about the QR reader library.