Layer not aligning depending on type of basemap

[RafaeldePedro]

Hi, so here's the issue I'm choking with. I apologise for trying to say it into so many works - if you see my two screenshots attached, they should speak a thousand words, but I leave the description of the problem below just in case. Hope someone can help:

Layers align with a GB Basemap

The red line layer does not align when Imagery basemap is selected

I've got a layer on ArcGIS Online to show site boundaries, shared from ArcGIS Pro, where it was created originally as a Feature Class in a GDB with Feature Dataset in British National Grid. The map in which this was created (and shared to ArcGIS Online) was in British National Grid and with transformation 'OSGB 1936 to WGS 1984 NGA 7PAR'. Not sure if this bit is important. When this layer and boundary were created in ArcGIS Pro, all features aligned as expected with other layers and any basemap of my choice (World Imagery, Open Street Map or any GB Base map).

Then, the map (containing only this layer) was shared as a Feature Layer to ArcGIS Online. But in this case, when I open the webmap, the Feature Layer behaved differently depending on the base map on display: when using a GB basemap (grey, topographic, cartographic, etc.) the boundary aligns with the other features as expected. However, when switching to other basemaps (e.g. Imagery), the boundary seems to have moved. I understand that GB basemaps and Imagery have different coordinate systems at their root (BNG v Web Mercator Auxiliary Sphere, I believe), so I think this must be the cause.

But, to make it even more complicated, there is another layer on the webmap. This is a shapefile containing some background vector lines that should align with OS features. This shapefile was created in ArcGIS Pro in a map with exact same properties as the map where the boundary feature class was published from. However, this shapefile has been added to ArcGIS Online as a zipfile, then added to the webmap.

Now, when we look at both layers on the map viewer, and play with the different base maps, what I find is that the zipfile shp layer behaves correctly regardless of the basemap, while the boundary layer (shared directly from ArcGIS Pro), only aligns properly on the GB basemap.

Any ideas, solutions, etc. please?

Thank you all

[JamesCarroll7]

Is it possible for you to go back to ArcGIS Pro, export your site boundary as a shapefile using the Feature Class to Shapefile tool (using whatever coordinate system you want, say BNG) and then import the shapefile to ArcGIS Online? That's the first thing I would try as it sounds like importing the other zipped shapefile which came straight from ArcGIS Pro worked, so in theory it should work if you made a new shapefile of your site boundary too. 

I use ArcGIS Online only very rarely so maybe someone with more knowledge than me can help, but I'd start with that, sometimes the simple fixes are the best.