Why do some multicellular eukaryotes still have magnetosomes?

A few facts I've researched from what is known:

• The magnetosome is a simple structure used for magnetoreception in some bacteria. It's also used for this in a few aquatic unicellular eukaryotes (protists like euglenids and algae).
• It consists of a linear chain of ferromagnetic magnetite crystals linked to the cell membrane and cytoskeleton which orients the cells parallel to the Earth's magnetic field, used for passive alignment and navigation.
• The magnetite (iron oxide) is produced on iron uptake by biomineralisation.
• The core genes and operons for the magnetosome are conserved across all bacteria they appear in, most of which are in phylum Pseudomonadota.
• All known magnetotactic bacteria live in anoxic waters. The Great Oxidation event in the Archaean eon likely provided the selective pressure for magnetosomes, as a way to store reduced iron (Fe⁰) to defend against reactive oxygen species (ROS).
• A few animals (e.g. migratory birds) have magnetoreception abilities too, but they work by a totally different mechanism (cryptochrome complexes). In the few multicellular eukaryotes where magnetosomes have been found (including humans, in our brain), they are all non-functional.

Why would we retain these magnetosomes? Could they really have stuck around for over a billion years since our days as a unicellular eukaryote or even a prokaryote pre-endosymbiosis, with no benefit? That seems extremely unlikely.

Thanks for any insights!