Update: emm...there's already an issue: flarum/core3043
As shown, after changing the subtag and refreshing, an error occurred (the tag became "deleted").
I have reproduced on a freshly installed test site, no 3rd party extensions are installed and default settings are kept.
Flarum core 1.2.0
PHP version: 7.4.20
MySQL version: 5.7.34-log
Loaded extensions: Core, date, libxml, openssl, pcre, sqlite3, zlib, bcmath, ctype, curl, dom, filter, ftp, gd, gettext, hash, iconv, intl, json, mbstring, SPL, session, pcntl, standard, PDO, mysqlnd, pdo_sqlite, Phar, posix, Reflection, pdo_mysql, shmop, SimpleXML, soap, sockets, sodium, mysqli, sysvsem, tokenizer, xml, xmlreader, xmlrpc, xmlwriter, zip, fileinfo, exif, Zend OPcache
+----------------------+---------+--------+
| Flarum Extensions | | |
+----------------------+---------+--------+
| ID | Version | Commit |
+----------------------+---------+--------+
| flarum-approval | v1.2.0 | |
| flarum-bbcode | v1.2.0 | |
| flarum-emoji | v1.2.0 | |
| flarum-lang-english | v1.2.0 | |
| flarum-flags | v1.2.0 | |
| flarum-likes | v1.2.0 | |
| flarum-lock | v1.2.0 | |
| flarum-markdown | v1.2.0 | |
| flarum-mentions | v1.2.0 | |
| flarum-statistics | v1.2.0 | |
| flarum-sticky | v1.2.0 | |
| flarum-subscriptions | v1.2.0 | |
| flarum-suspend | v1.2.0 | |
| flarum-tags | v1.2.0 | |
+----------------------+---------+--------+