G3.0.3 [3081], macOS 10.13.6 – Not a new issue, has been happening for past builds since Stem Thickness was updated for G3/Py3.
What is happening is that when two files are open at the same time, one can switch between the two files with no change in Stem Thickness function.
However, after working for some time (perhaps an hour or so) switching to the reference file to do some small thing such as copy an element or measure something with Stem Thickness then returning to the work file, Stem Thickness doesn’t function dynamically, i.e. when the cursor moves it will measure only on a mouse click. When this happens, deactivating Stem Thickness then reactivating it will restore it to normal.