select * from problem;

shows there is a problem record in the "problem" table, e.g.

 64 | 3 | 0 | 15693 | 1530789636 | 973867100 | | 0 | 0 | | | Cannot evaluate expression: "Cannot evaluate function "Windows:eventlog[testlog].logsource(^abc$\)": invalid regular expression.". 

In Problems screen filter I can select a trigger with Status="Unknown", press "Apply" and see nothing.
Expected result: information about which trigger and why is in "Unknown" status, because the information is recorded in "problems" table.
Yes, I can go to "Configuration->Hosts-><host>->Triggers" screen and filter to see triggers in "Unknown" state to fix it, I can configure notifications about "Unknown" triggers... But to find out is everything ok, I turn to "Problems" screen... and problem is not there.