The NIP for ephemeral events (incl. some used in NWC flow) was recently updated, not all relays respect that. Some SDKs expect the new behavior, which leads to weird bugs when using older non-compliant relays.
See more details here: https://github.com/rust-nostr/nostr/issues/164#issuecomment-1786049227
To be backward compatible and handle both types of relays, make sure to not expect an "OK" response after sending an ephemeral event.