It is only happening in the default “Location” field for me, 2.9.0.beta7( 0760b249ff ).
I tried saving an emoji as well, but upon saving and refreshing the page, only the Location field is exploding the URL.
<a href="https://meta.discourse.org/" class="onebox" target="_blank" rel="noopener nofollow ugc">https://meta.discourse.org/</a>
It appears to be one-boxing, so that’s a hint. 