I was having the exact same issue, and finally figured this out via trial-and-error. The issue is that there is a bug in the Windows 10 Path environment variable parser. In my case, the first entry in my Path environment variable started with an application-defined REG_SZ environment variable (e.g., Path was set to
%MyAppPath%\some\subdir;...). From my testing, it appears that the first entry on the Path must not start with a non-built-in environment variable.
For example, if my Path environment variable were set to
%SystemRoot%;%MyAppPath%\some\subdir;..., it would be editable in the Path list view editor, since
SystemRoot is a built-in environment variable. If you swap the first two paths in that example, I observed the symptom you describe (that editing the Path variable results in opening the standard text editing dialog instead of the Path list editor).
To fix this issue, ensure the first entry in your Path doesn't start with a non-built-in environment variable.
%SystemRoot%\System32(the third entry) to the front of your path, and this issue will be resolved. Note that in your first screenshot, the background dialog displays the path as
%M2_HOME%..., whereas the second displays the path as
C:\Users\.... That indicates that
USERPROFILEis a built-in Windows environment variable, but
M2_HOMEis not (This can be verified by observing USERPROFILE isn't listed in HKCU\Environment). See my answer for more details.