"Marking" field on the Planned Orders form

Hi everyone, I’m sorry if this issue has already been discussed in this forum but I tried searching and couldn’t find anything. Ok this is the problem I’m faced with: Under Master planning >> Master planning Area >> Common Forms >> Planned orders >> Overview tab. When I select a record by ticking a checkbox from the Marking field and maybe just highlight or just select any record below or above it(without ticking it’s check box), and then clicking the Functions button. Both selected records will have a mark or a tick under the Marking field, even the one that I didn’t tick or check. Meaning, the other checkbox gets marked/checked automatically just by virtue of highlighting the record. How do I fix this ? Anyone with a solution please help,

Thanks in advance.