The TOTP feature in Bitwarden works, if you paste in the whole otpauth:// URI to Bitwarden’s Authenticator Key (TOTP) field. The URL specifies that the hashing algorithm should be SHA256. If you just import the secret= value into Authy, it probably defaults to using the SHA-1 algorithm, which may be why the codes generated by Authy don’t work.
SHA256 is more secure than SHA-1, which I guess is why Lemmy has chosen to use it for its 2FA feature.