There’s no evidence it’s a bug in opensmtpd, rather it’s that other SMTP clients such as WhatsApp’s mail servers are assuming support for the (non-mandatory) PIPELINING extension instead of respecting the EHLO-advertised (lack of) support.
That is exactly why I would enable debugging or tracing and see who is sending what and whom is responding or waiting on something expected with timestamps so that nobody is guessing. The tracing should occur on both the sending and receiving side.
