iPhone camera consolidating geotags for multiple photos
Check the GPS time on your photos. This is the time of the GPS fix used to tag your photos. I.e. the time at which iPhone determined your location. It is possible that your iPhone failed to get a GPS fix while you were photographing. In that case it can decide to use the last known location. You would find that all your photos share the same GPS time.

Many possible reasons: High buildings, narrow streets, etc. can sometimes impair GPS reception. The iPhone GPS could have had a software problem that was later resolved by restarting the phone. Your iPhone may have been on low power mode.

I recently hiked Kilimanjaro and had set my iPhone to low power mode while on the mountain. It seems this had the effect of iPhone powering up the GPS much less frequently. I have photos where the GPS fix predates the photo by several hours. 

I also saw what I consider to be a bug: iPhone seems to have discarded more recent GPS fixes in favor of better (but older) fixes. I have a few situations where one photo has a rather recent GPS fix and photos taken hours later were tagged with a GPS fix from the previous day!