I am 100% sure I have never encountered any problems when using a PC except asking for an email address or telephone number for verification but you can skip that by using different IP addresses via VPN. I have never tried creating a Proton account using a mobile phone therefore cannot comment on the issues you faced. Are you using a VPN?
You probably had an account with them before clicking the link in question. I'm still unsure if it's limited to new users or just happening to me.