A lone master with semisync plugins loaded and enabled (both master & slave, slave likely not required) will sometimes wait for slave confirmation even when it shouldn't. Tested with following settings and variables:

SHOW status LIKE 'rpl_semi%';
Variable_name |Value |
------------------------------------------|-----|
Rpl_semi_sync_master_clients |0 |
Rpl_semi_sync_master_net_avg_wait_time |0 |
Rpl_semi_sync_master_net_wait_time |0 |
Rpl_semi_sync_master_net_waits |0 |
Rpl_semi_sync_master_no_times |1 |
Rpl_semi_sync_master_no_tx |1 |
Rpl_semi_sync_master_status |OFF |
Rpl_semi_sync_master_timefunc_failures |0 |
Rpl_semi_sync_master_tx_avg_wait_time |0 |
Rpl_semi_sync_master_tx_wait_time |0 |
Rpl_semi_sync_master_tx_waits |0 |
Rpl_semi_sync_master_wait_pos_backtraverse |0 |
Rpl_semi_sync_master_wait_sessions |0 |
Rpl_semi_sync_master_yes_tx |0 |
Rpl_semi_sync_slave_status |OFF |

SHOW variables LIKE 'rpl_semi%';
Variable_name |Value |
----------------------------------|------------|
rpl_semi_sync_master_enabled |ON |
rpl_semi_sync_master_timeout |1500 |
rpl_semi_sync_master_trace_level |32 |
rpl_semi_sync_master_wait_no_slave |OFF |
rpl_semi_sync_master_wait_point |AFTER_COMMIT |
rpl_semi_sync_slave_enabled |ON |
rpl_semi_sync_slave_trace_level |32 |

SELECT VERSION();
VERSION() |
-------------------------------------------|
10.2.18-MariaDB-1:10.2.18+maria~bionic-log |

Commands which trigger the wait:
RESET MASTER; FLUSH TABLES; (a lone FLUSH TABLES does not trigger the wait)