It turned out that if trigger expression has a space(s) before and/or after an operator sign (<>#=) - this trigger will be not shown in a graph legend.

Will be shown:

{host:itemkey.last(0)}

>444

Will not:

{host:itemkey.last(0)}

> 444

It's absolutely unclear what's going on, should be fixed.

Yes, many users are using spaces there when writing/editing expression manually, and it's not user's mistake, such expression is supported.