I have experienced this on a regular basis in the last month. I have an iPhone SE and my husband has an iPhone 11, both up to date on everything. Poor cell signal issues aside, when I try to see my husband's phone in "Find My" app, it says "location not available." If I persist, it may eventually give me an incorrect location that is (estimated) 1-3 miles away from his actual location, and does not refresh. Yesterday we tested it. We were together at a location. His phone said both of us were at that location. My phone said I was at that location, but that he was about 2 miles away. Also, the accuracy circle for his location did not include the actual location, which was about 1 mile outside the circle. Until about a month ago, locating my husband's phone worked as well as can be expected.
I think I will report a problem to Apple Feedback.