Reliability of TimerTrigger

#### Is your question related to a specific version? If so, please specify:
Version 2.0
#### What language does your question apply to? (e.g. C#, JavaScript, Java, All)
C# with Microsoft.NET.Sdk.Functions-1.0.29
#### Question
Are time triggers reliable? I've a fuction with the following trigger:
[TimerTrigger("00 */30 8-19 * * 1-5")]

So it should be executed every 30m, but as you can see in the following image, it skipped 4 executions:
-9:00
-9:30
-12:30
-13:30


Start with the TimerTrigger entry point and the schedule expression "00 */30 8-19 * * 1-5" under C# with Microsoft.NET.Sdk.Functions-1.0.29, then review the execution-history image and the reported 9:00, 9:30, 12:30, and 13:30 omissions. Done means explaining whether these skipped executions are expected or identifying the reliability issue and its conditions.

Written by the indexing model from the issue text.