Hi @PEspo,
I have made a test on my side to sign in flow in my phone with IOS system and don't have the issue that you mentioned.
Please make sure you have sign up an account of microsoft flow and have a good net work when you sign in the account.
And please make sure you have fill in the correct password to sign in the account.
Also please take more try to sign in the account.
Best regards,
Alie