The mariabackup --prepare shows this warning:
2021-10-07 1:13:23 0 [Note] InnoDB: Starting a batch to recover 9962 pages from redo log.
2021-10-07 1:13:28 0 [Warning] InnoDB: Difficult to find free blocks in the buffer pool (21 search iterations)! 21 failed attempts to flush a page! Consider increasing innodb_buffer_pool_size. Pending flushes (fsync) log: 0; buffer pool: 0. 582296 OS file reads, 504266 OS file writes, 2396 OS fsyncs.
The error is similar to:
But looks like they are already fixed in 10.5.1