I've configured sending notification of problems as described in the documentation.
Next, I create an emergency - the trigger generates a problem. And the notification is sent and I receive it successfully.
After a while, I repeat the experiment - the trigger fires, but the notification does not send.

That's what I could dig.
1. An entry in the escalations table is created. Nextcheck = 0.
2. Server logs
Code:
28139:20200228:161434.487 In process_escalations()
28139:20200228:161434.487 query [txnlev:0] [select escalationid,actionid,triggerid,eventid,r_eventid,nextcheck,esc_step,status,itemid,acknowledgeid from escalations where triggerid is null and itemid is not null and nextcheck<=1582899277 order by actionid,triggerid,itemid,escalationid]
28139:20200228:161434.488 End of process_escalations()
28139:20200228:161434.488 In process_escalations()
28139:20200228:161434.488 query [txnlev:0] [select escalationid,actionid,triggerid,eventid,r_eventid,nextcheck,esc_step,status,itemid,acknowledgeid from escalations where triggerid is null and itemid is null and nextcheck<=1582899277 order by actionid,triggerid,itemid,escalationid]
28139:20200228:161434.489 End of process_escalations()
3. After restart zabbix server - a notification is sent.

I conclude that the process that processes the escalations table does not see the new record.

I think, that problem can be in auto_commit=0 (MySQL), which is in my config. Buy i changed it to 1 and problem also present.