Values are good with the following commands :

MariaDB [nextcloud]> SHOW GLOBAL VARIABLES LIKE "innodb_buffer_pool_chunk_size";
innodb_buffer_pool_chunk_size | 805306368 |

MariaDB [nextcloud]> SHOW GLOBAL VARIABLES LIKE "innodb_buffer_pool_size";
innodb_buffer_pool_size | 6442450944 |

MariaDB [nextcloud]> SHOW GLOBAL VARIABLES LIKE "innodb_buffer_pool_instances";
innodb_buffer_pool_instances | 8 |

Values are not good when we check with the command "show engine innodb status\g" :

BUFFER POOL AND MEMORY
----------------------
Total large memory allocated 6623854592
Dictionary memory allocated 403920
Buffer pool size 393216
Free buffers 342904

INDIVIDUAL BUFFER POOL INFO
----------------------
---BUFFER POOL 0
Buffer pool size 49152
Free buffers 42853
Database pages 12368
Old database pages 4581

The value of each instance is 49 MB instead of 768 MB