There is some forum post about this one but I was unable to find it. This is a bug that should have been fixed in 6.10.2 (see the release notes of that one). If you still experience this problem in a later version create a support ticket with a demo project showing the bug because this should not be happening.
And may be you could solve it in an on change microflow that chechs this that and set it empty if it is so.
Found the post. See here: https://forum.mendix.com/link/questions/85994