Had my map in dark mode by changing the default tile url to the following: https://server.arcgisonline.com/arcgis/rest/services/Canvas/World_Dark_Gray_Base/MapServer/tile/{z}/{y}/{x}, one of those mentioned in this post: Perspective Map "Theming" - Internet Tile Server Options.
Maybe by using the default map ( which is mostly white ) you don’t see the small white parts in the corners when rounding, or as @victordcq says, probably the map doing something weird.
I added an empty label above and beneath the map, and it seems that the white corners are gone ( see the below figure, implying that it’s the map doing something weird ).
If I remove the labels, it’s back to the white corners:
With the default map tile it’s hard to tell if there is some white corners “going on”:
