There are regulations to specifically address the requirement.
A test is a moment in time.
I’ll relate a couple more anecdotes and you can decide for yourself if it’s considered 100% reliable by others.
Newer devices like the AFDD combine earth leakage protection with Arc fault detection in the same unit.
These recent designs have a self-test facility which is specified by regulation to self-test a minimum of once every 24hrs.
However, I note that the smarter manufacturers run the self-test every 15hrs to ensure a daily offset so that the test happens at different times every day, This ensures the device is tested during the range of conditions that it will endure.
Here is another little factoid that also demonstrates an earth leakage can be temporarily blinded:
Robin loop testers have a patented design that injects DC during certain tests so that can be done without disconnecting the standard earth leakage. (Which would otherwise trip during the test).
For more info google: Robin De-lock Loop Tester
Or:
I understand that this functionality still blinds a Type A earth leakage.