Line join looks strange in Web Map

[HanZheng]

I have a web map with offset symbol. After I published the map, the offset solid strokes display properly, but cap and join of lines are not rendered properly. I tried different mode of join and cap none of which gives me lines that are properly connected like that in ArcGIS Pro. The cap and join types I currently use are square and round

What it looks like in ArcGIS Pro

What it looks like in Web Map, A triangular shape object is attached to line join.