Point of Interest icon stays white after exploring
Solution 1:
In my experience the Remnant Site icons for the Vaults never changed to blue, even after completing them. For other Remnant Sites they seemed to change after opening one or containers at the site.
The exception to this rule for me was on Eos, where two sites never changed to blue. Perhaps there was a bug that was patched after I discovered these sites, which did not correct them.