I have replicated the same issue on a second VPS from a different provider, and have tried using both PHP 8.3 and 8.2 -- the same basic problem occurs. Both VPSes are fresh installs of Debian 12, using Hestia CP. Search worked perfectly for a year previously.
Flarum core: 1.8.7
PHP version: 8.3.12
MySQL version: 10.11.6-MariaDB-0+deb12u1
Loaded extensions: Core, date, libxml, openssl, pcre, zlib, filter, hash, json, pcntl, random, Reflection, SPL, session, standard, sodium, mysqlnd, PDO, xml, bcmath, bz2, calendar, ctype, curl, dom, mbstring, FFI, fileinfo, ftp, gd, gettext, iconv, imagick, imap, intl, ldap, exif, mysqli, pdo_mysql, Phar, posix, pspell, readline, shmop, SimpleXML, soap, sockets, sysvmsg, sysvsem, sysvshm, tokenizer, xmlreader, xmlwriter, xsl, zip, Zend OPcache
+------------------------------------------+---------+--------+
| Flarum Extensions | | |
+------------------------------------------+---------+--------+
| ID | Version | Commit |
+------------------------------------------+---------+--------+
| flarum-tags | v1.8.2 | |
| fof-merge-discussions | 1.4.1 | |
| flarum-flags | v1.8.1 | |
| v17development-seo | v1.8.1 | |
| nearata-tags-color-generator | v2.3.0 | |
| nearata-embed-video | 3.3.2 | |
| michaelbelgium-discussion-views | v7.3 | |
| katosdev-automore | 1.0 | |
| ianm-synopsis | 1.3.4 | |
| fof-recaptcha | 1.3.4 | |
| fof-links | 1.2.3 | |
| fof-html-errors | 1.1.2 | |
| fof-forum-statistics-widget | 1.2.1 | |
| fof-formatting | 1.0.3 | |
| fof-discussion-thumbnail | 1.1.3 | |
| flarum-suspend | v1.8.2 | |
| flarum-statistics | v1.8.1 | |
| flarum-mentions | v1.8.4 | |
| flarum-markdown | v1.8.1 | |
| flarum-lock | v1.8.1 | |
| flarum-likes | v1.8.1 | |
| flarum-lang-english | v1.8.0 | |
| flarum-emoji | v1.8.1 | |
| flarum-com-tags-under-discussion | 1.1.0 | |
| flarum-bbcode | v1.8.0 | |
| flarum-approval | v1.8.2 | |
| davwheat-custom-sidenav-links | 1.0.1 | |
| datlechin-link-preview | v1.5.0 | |
| datlechin-discussion-count | v0.1.0 | |
| clarkwinkelmann-selective-mediaembed | 1.0.1 | |
| clarkwinkelmann-formatted-banner | 1.0.0 | |
| clarkwinkelmann-discussion-open-position | 1.1.0 | |
| clarkwinkelmann-circle-groups | 1.0.1 | |
| clarkwinkelmann-author-change | 1.0.3 | |
| blomstra-sort-order-toggle | 0.2.0 | |
| blomstra-fontawesome | 0.1.5 | |
| askvortsov-categories | v3.1.0 | |
+------------------------------------------+---------+--------+
Base URL: https://[redacted]
Installation path: /[redacted]
Queue driver: sync
Session driver: file
Mail driver: smtp
Debug mode: off