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.
Regards,
Ronald
[EDIT]
And may be you could solve it in an on change microflow that chechs this that and set it empty if it is so.
[EDIT2]
Found the post. See here: https://forum.mendix.com/link/questions/85994