@lmashton Hello, recently I too got into this bug for my newer posts, So I manually added altand caption tags in Text mode, it is fine now. Try this, hope it works.
Check this,
[caption id="attachment_954" align="alignnone" width="673"]<img class="wp-image-954 size-full" src="https://site.com/wp-content/uploads/2018/03/domain_to_IP.png" alt="your alt" width="673" height="292" />your caption[/caption]
