BNG Layers offset when posted online

[Nicauldron]

Hi there

I have noticed that when I post data in the BNG projection online, they are offset slightly. 
I solved this by either using a BNG basemap from AGOL layers or projecting the data to WGS84 or Web Mercator before I post it.
However, I thought that the data was meant to project itself on-the-fly when online? 
Is there a way to fix this or should I keep projecting all my data before I post it online?
In the case of the latter, what projections does everyone suggest I use for UK Data (England, Wales and Scotland)

Here is a snip of one of my boundary line offsets. You can see the line is meant to follow the tree line here. Before posting online, it looks perfect in ArcPro.

[berniejconnors]

Nicauldron,

        I don't publish map services very often any longer.  That job has been passed to one of my co-workers.  But I do recall there were several transformation methods to go between ours CRS (EPSG 2953) and WGS84.  If the wrong method was used there would be a ~ 1 metre shift when our data was combined with other data in a Web map.  To prevent this from happening I had to define the preferred transformation in the properties of the MXD I was using to publish the map service.  I expect there is a similar capability for ArcGIS Pro.

        Here is a discussion of the NAD83 to WGS84 transformations that may help you decide if a similar problem exists for the BNG - https://desktop.arcgis.com/en/arcmap/latest/map/projections/choosing-an-appropriate-transformation.h...

Best regards,

Bernie.