Why is the Page Life Expectancy (PLE) not increasing?

Question

We have monitored the PLE performance counter for several SQL Server 2016 instances for over a year, and have had alerts which triggered when this value persistently dropped to zero or froze.

Recently we upgraded to have two nodes clustered via SQL AlwaysOn, but have noted the following behaviour:

1. Typically both clustered nodes have their PLE increase normally regardless of which is the active member.
2. Occasionally one of the nodes may have its PLE freeze at a static value for hours or days, after which it resumes counting, or drops to zero before resuming.

Should we be following any actions or investigation if the PLE remains static for either node? Is this normal PLE behaviour for AlwaysOn clustered nodes?