See this KB article for information about GPS support on the R5 using GP-E2 or smartphone.
@Atticuslake brought up some good points.
GPS (tagging) reliability is only as good as the signal and connection to / from the smartphone.
-GPS aquisition can take time and accuracy isn't always stellar. Geo-Tagging is most reliable when used consistently, from a stationary location. If you are moving, accuracy can diminish. When you use a smartphone, the camera must mantain a constant connection for GPS accuracy. If either device goes to "sleep" or you move, location accuracy can once again diminish. The aqusition process starts again each time the connection between the the devices is re-established.
We aren't saying this is what's happening in your situation, but that the possibility exists and might explain why you are experiencing inconsistent results.
The GP-E2 is likely to work better than your smartphone. I really appreciate this feature which is supported natively on my body. Canon appears to recognize this as built in support is again present on the R3. I am hopeful it will be present on the R5c (when released) as well. Some peope don't care about geo-tagging. I use it for every shot.
Your phone is providing the latitude and longitude. As far as the camera is concerned, this is just a string of bits that gets stored in the picture file. It's highly unlikely that the camera is changing the data. So I see two possibilities:
Both are possible, but 2 is certainly worth looking into. GPS signals are extremely weak, and if the phone can't update itself, it will continue to use the last location it got -- or even fall back on cell tower location, which is very poor. (Of course I assume your phone's GPS is actually running, and that the Canon app on the phone has permission for it).
So if your phone has poor GPS signal, that could explain things. For example, if it's in your pocket, or you're among trees or buildings. Again, GPS is extremely weak. I've certainly run into exactly this issue just because of trees -- in my case, it was my smartwatch that was getting an out-of-date and wildly inaccurate location from the phone.
Run a mapping app of some kind (e.g. Google maps) on your phone, and see where it tells you you are.