sec-fetch-mode headers missing for OAuth

[essential-randomness]

Hello,
I received a bug report where someone on ios 15.8.3 had trouble with OAuth login because of the lack of a sec-fetch-mode header. I suspect this is due to iOS not supporting the header until 16.3. Is there a fix from the OAuth implementer side, should this be fixed on BlueSky's side, or is it working as intended?

Thank you!