Why queries in cache cleaned every 30 mins?

I try to figure out, why my mariadb instance clean queries in cache. It happens every 30 minutes. It doesn't happen by cron or event scheduller. What i'am missing? Some mariadb settings?

MariaDB [(none)]> SHOW STATUS LIKE 'Qcache%';
+-------------------------+-----------+
| Variable_name           | Value     |
+-------------------------+-----------+
| Qcache_free_blocks      | 1471      |
| Qcache_free_memory      | 219999872 |
| Qcache_hits             | 203798549 |
| Qcache_inserts          | 65763537  |
| Qcache_lowmem_prunes    | 3758579   |
| Qcache_not_cached       | 1689178   |
| Qcache_queries_in_cache | 16441     | - flushed every 30 mins
| Qcache_total_blocks     | 35341     |
+-------------------------+-----------+

mariadb: mariadb Ver 15.1 Distrib 10.3.38-MariaDB, for debian-linux-gnu (x86_64) using readline 5.2 linux: Ubuntu 20.04.6 LTS