I have noticed same. We also share locations on iPhones and Normally, 95% of the time, it is accurate to GPS levels. I have used it to find her in a huge motorway service station car park before now, just by walking to the place where my own location dot on the map coincides with her dot, worked a treat.
Occasionally, it seems to become hugely inaccurate or sometimes just fails to locate at all. I have never figured out why, but it tends to happen when the phone has been indoors for a while, and/or been stationary in same place for a while.
Obviously, a weak GPS signal might make location less accurate. And if the phone is not on wi fi, and is in a mobile data black spot, obviously it can’t report its location at all. But without going into too much tedious detail, I have analysed specific symptoms and discounted both of these explanations, I am convinced it is just buggy.
When I track her while driving her car, it always seems totally accurate. And I have never known either of our phones to get its own location inexplicably wrong when using maps Apps on the same device, it is just the location shared with other devices that seems occasionally to be very wrong.