The field in question is of composite system class name type:
Steps to Reproduce
Log in to a demo OOB instance.
Open a list filtered as follows:
Note that the magnifying glass icon next to the personalize list (gear) icon is disabled and the Task Type column is read-only.
Modify the filter as follows
Note that the search icon is now enabled and Task Type can be edited.
Remove the combined condition OR/AND involving Task Type.
Note – After carefully considering the severity and frequency of the issue, as well as the the cost and risk of attempting a fix, a decision has been made not to address this issue in any current or future releases. We do not make this decision lightly and we apologize for any inconvenience. If you have any questions regarding this problem or incident, please contact Customer Support.
Related Problem: PRB645541