That's because Zabbix is unable to evaluate agent.ping.max(300) since there is no data in the database.
Change order of the conditions to make it work:
{Template_Linux:agent.ping.nodata(300)}|
{Template_Linux:agent.ping.max(300)}=0
I tried changing, it didn't help. That's the issue.
For the record, this is what I resorted to, works fine:
{Template_Linux:agent.ping.count(300,1)}=0 |
{Template_Linux:agent.ping.nodata(300)}Hi,
Аll trigger functions shall be evaluated without errors. Otherwise the status of the trigger becomes UNKNOWN.
I'm closing the issue.
also, why would you do it in such a convoluted way ?
agent.ping can only return 1, so checking that 'max' = 0 would never work at all.
just use nodata(300) 
>Аll trigger functions shall be evaluated without errors. Otherwise the status of the trigger becomes UNKNOWN.
Well, you just reworded the very issue. I can evaluate the expression, you can, just about anyone can. But Zabbix cannot.
>also, why would you do it in such a convoluted way? agent.ping can only return 1, so checking that 'max' = 0 would never work at all.
Makes sense. In this particular use case.
Тhe trigger never to return to OK state if one function can't be evaluated.
1.
{<evaluation failed>}=0 | 1 <-- trigger in PROBLEM state (it is logical)2. {<evaluation failed>}
=0 | 0 <-- Zabbix shall consider result of the first function, but can't
1. It would be logical if it was so. But it isn't. Trigger doesn't flip to PROBLEM. There's no alert sent.
2. Yes, that's right.