I can't replicate on the beta website.
It doesn't happen with every discussion but only with this specific one (AFAIK, since I can't check all of them).
The discussion might (?) be particular because it was a bit like a "welcome" discussion, where many people said hello when they signed up. The discussion is a sticky one, there are some posts that have been split in new discussions, and it's been locked/unlocked multiple times in the past.
Flarum info:
Flarum core 0.1.0-beta.8.2
PHP version: 7.2.17-0ubuntu0.18.04.1
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, Zend OPcache
+-------------------------------+-----------------+------------------------------------------+
| Flarum Extensions | | |
+-------------------------------+-----------------+------------------------------------------+
| ID | Version | Commit |
+-------------------------------+-----------------+------------------------------------------+
| flarum-bbcode | v0.1.0-beta.8 | |
| flarum-emoji | v0.1.0-beta.8 | |
| flarum-flags | v0.1.0-beta.8.1 | |
| flarum-likes | v0.1.0-beta.8.1 | |
| flarum-markdown | dev-master | e11a86001396aff7322f7b60aee614234daadd41 |
| flarum-sticky | v0.1.0-beta.8 | |
| flarum-suspend | v0.1.0-beta.8 | |
| flarum-tags | v0.1.0-beta.8.2 | |
| flarum-approval | v0.1.0-beta.8 | |
| flarum-mentions | dev-fc8 | ad509a766e73f14b8bb04e13d0b33c86d315f47b |
| flarum-subscriptions | dev-fc8 | 83964f639e54276167b58153e6d4e97da5b0a4f4 |
| flarum-lang-english | v0.1.0-beta.8 | |
| flarum-statistics | v0.1.0-beta.8 | |
| flarum-auth-github | v0.1.0-beta.8 | |
| flarum-auth-facebook | v0.1.0-beta.8 | |
| flarum-auth-twitter | v0.1.0-beta.8 | |
| flarum-pusher | v0.1.0-beta.8.1 | |
| flagrow-analytics | 0.8.0 | |
| fof-links | 0.2.0 | |
| fof-user-bio | 0.1.2 | |
| clarkwinkelmann-emojionearea | 0.2.0 | |
| flagrow-ads | 0.2.0-beta.1 | |
| botfactoryit-italian | dev-master | f2c3e604e35e5676f4c24d9dd8a7b1f9a4d473c8 |
| flagrow-impersonate | 0.2.0 | |
| flagrow-sitemap | 0.2.0 | |
| fof-share-social | 0.1.1 | df0217f4f2080d978059fa4406454ef63eb1b1a9 |
| fof-pages | 0.2.3 | |
| fof-disposable-emails | 0.1.0 | |
| botfactoryit-upload | dev-master | 3b5d8c03d773c398fa6798259df89dc0f3008672 |
| flagrow-amazon-affiliation | dev-master | 82438b92a63c096b32b88189c78d338bf7ecea52 |
| clarkwinkelmann-circle-groups | 0.2.1 | |
| v17development-seo | 1.0.1 | |
| fof-split | 0.4.3 | 53b163f0e5043bb014a35a99dd834042148babe4 |
| botfactoryit-fibraclick | dev-master | bc4f9f8d9f84a575cb1b790d466f50cd51830891 |
| flarum-lock | v0.1.0-beta.8 | |
+-------------------------------+-----------------+------------------------------------------+
Base URL: https://forum.fibra.click
Installation path: /usr/local/openresty/nginx/www/fibraclick-forum
Debug mode: off
EDIT: "flarum-lock" is at the bottom probably because I disabled and re-enabled it to see if it changed something...
EDIT2: the markdown, mentions, subscriptions extensions point to a commit because I forked them to do some minor changes like translating some strings