This is Dashlane support answer:
I understand from your message that you need to enter your Master Password when using a passkey on some websites that require extra verification, like user verification (UV). For example, Discourse requires this extra step during login with a passkey. That’s why Dashlane asks for your Master Password or biometrics, if set up, to confirm your identity. Other sites might not have this requirement, which is why you only see it on certain platforms like Discourse.
So from that answer, I understand that Discourse sites do require the extra step of asking Dashlane master password when using passkeys (which is worst than using passwords because using Dashlane stored passwords do not require typing the master password), that other non Discourse sites might not.
In summary, using passkeys with Dashlane for Discourse is not recommended, better use passwords.