You are not the only one to notice this issue.
Do you use Web Mercator or WGS84 for your basemap or something else? I think the Measurement Widget was only designed for Web Mercator/WGS84.
If you would like a Measurement Widget that can handle any coordinate system, this one was designed for it. https://community.esri.com/t5/experience-builder-custom-widgets/advanced-draw-widget-improvements-im...
I have noticed this. Area measurements are also off. Basically, all forms of interactive measurement, whether through the Measurement widget or the the Draw widget can not be taken seriously. Taking measurements in a GIS is such a core function and the ExB tools to do so are very primative and inaccurate.
I used to complain there was no way to log or label individual segments on a multi segment measurement, as if that was a big deal, now I don't even believe simple measurements are trustworthy and asking for a more complex measuring tool is asking for more issues. This is a shame because I build web apps for County appraisers who work with cadastral data, surveys, metes & bounds legal descriptions, deed work, etc every day and they have no way to "follow along" such material on the mapping, or even take a measurement we can believe in.
Suspect it is because you are using Web Mercator for you maps as it is known to throw 'inaccurate measurements', especially the farther away you get from the equator.
My ExB app(s) are all in State Plane, and I get about as close of measurements as I can without ExB supporting snapping in the measure tool.
R_
I have posted the following several times:
I have a web map in the my local stateplane projection. When using the Draw widget tool tips and constraits, the "total length" (or total area if drawing polygon) does not match the label that is placed when Show area measurement is checked on.
I don't think the issue is with the basemap coordinate system, I think the issue is with the measurement widget and draw widgets. You can't trust either!