It is expected for the Temporal Membership to behave this way.
This is because the read-only value of “edsva-ScheduledOperation-PollingInterval” is set to 30 minutes and is driven from code.
The scheduler for Temporal Membership will happen based on the edsva-ScheduledOperation-PollingInterval value. Therefore, it is as per design that Temporal Membership will update after every 30 minutes.