I presume you also followed https://auth0.com/blog/securing-gatsby-with-auth0/ like myself. I too have an issue with iPad and auth0 using a custom domain. The callback page just doesnt grab that accessToken.
Been remote debugging using Safari usb to the iPad. Strange, if i run gatsby with: gatsby develop -H 0.0.0.0
And then look at my local IP (eg, 192.168.15.7:8000) and add that to the auth0 callback and web origin i can then bring up the local network hosted website on my iPad to the locally running gatsby app. I also connect using Safari remote inspector. All works just fine, i get my access token and silent auth all work fine.
Deploy online… to (for example) mywebsite.com with a custom auth0 domain of auth.mywebsite.com
auth.parseHash(setSession()) just always returns null.
But. Revert the authjs code to call the auth0 domain instead of my custom eg, mytenant.auth0.com and it works perfectly.
For more info i have another domain (eg anotherwebsite.com) that uses the same client, this works on iPad fine using either the custom auth0 mapped domain or the provided auth0 domain.
So my intuition is telling me its something to do with logging in (on iPad only os 13.2.2) when you are calling from the root domain that also matches part of the custom auth0 domain.
mywebsite.com calling auth.mywebsite.com no access token
mywebsite.com calling mytenant.auth0.com gets access token
anotherwebsite calling either auth.mywebsite.com or mytenant.auth0.com gets access token