Why am I getting buffers of different radius when using ArcGIS Pro to buffer and post to ArcGIS Online then when I create the buffer on ArcGIS Online?

[deleted-user-6g41xWiDoeRO]

I created 50m buffers on ArcGIS Online from field collected line features to assess an area of treatment (blue buffer in attached image). In the map viewer they measure out accurately to 50m. After the field crew added length to one of the features, I used ArcGIS Pro to create a 50m buffer on the new line by pulling the data from my ArcGIS Online Portal (green buffer in attached image). When I uploaded the buffer created in Pro to my portal and added it into my map I noticed the distances were significantly different. When measured in the map viewer, the buffer created using ArcGIS Online (blue) measures 50m while the buffer created in Pro measures ~38m. I pulled the 2 buffers back into Pro from my ArcGIS Online Portal and re-measured which produced the results of the ArcGIS Online buffer (blue) being ~66m while the one I created in Pro measures 50m. My map is projected in WGS 1984 Web Mercator Auxiliary Sphere along with the buffer that I created in Pro. Some clarification as to why this happened/ which data is correct would be appreciated.

[DanPatterson_Retired]

Web Mercator, isn't the best projection do the buffering.  WM has 'issues' with anything involving distance, and area measurement.  Pro has a better chance of handling coordinate conversion.  It would be best to do your buffering in Pro since it handles planar and geodesic

Buffer—Help | ArcGIS Desktop 

[deleted-user-6g41xWiDoeRO]

I created new 50m buffers using UTM and they match up with the ones I created in ArcGIS Online while also measuring correctly after changing my map projection to the proper UTM zone. I thought ArcGIS Online used WM projections so that seemed like the correct course of action to replicate the buffers I created in the map viewer. Thanks for your help!