I seem to be experiencing this issue, even though I believe flarum/mentions is on the latest commit for me:
php flarum info
returns this:
Flarum core 0.1.0-beta.8.1
PHP version: 7.3.0-1+ubuntu18.04.1+deb.sury.org+1
Loaded extensions: Core, date, libxml, openssl, pcre, zlib, filter, hash, pcntl, Reflection, SPL, sodium, session, standard, mysqlnd, PDO, xml, calendar, ctype, curl, dom, mbstring, fileinfo, ftp, gd, gettext, iconv, json, exif, mysqli, pdo_mysql, Phar, posix, readline, shmop, SimpleXML, sockets, sysvmsg, sysvsem, sysvshm, tokenizer, wddx, xmlreader, xmlwriter, xsl, zip, Zend OPcache
+---------------------+-------------+------------------------------------------+
| Flarum Extensions | | |
+---------------------+-------------+------------------------------------------+
| ID | Version | Commit |
+---------------------+-------------+------------------------------------------+
| flarum-lang-english | dev-master | |
| flarum-mentions | dev-master | 743f3f19ea2d4999bfaea4e38476d8ed3df0094e |
+---------------------+-------------+------------------------------------------+
Base URL: http://flarum.localhost
Installation path: /home/ordinaryjellyfish/Documents/flarum
Debug mode: ON
Don't forget to turn off debug mode! It should never be turned on in a production system.