I noticed this error after it had been running for quite some time. I had a server on my LAN that I used mainly as a test server to do a lot of testing before going live with any website. For flarum, I found that the mentions extension seems to have a problem that the domain name is not updated. When you have a server on, for example, 192.168.1.4, and then after you set up the extension mentions, move the entire website to 192.168.1.6. At this time, regardless of the Existing or newly generated ping(@) will use 192.168.1.4 as the domain name. The current version where this problem occurs is v1.8.2. I did not find a similar topic in the forum, and I did not see any commit or pull request related to the fix in github.
Re-enabling the plugin fixes this issue. This error is difficult to notice, which may cause problems such as IP/domain name leakage.
