Limit space taken by Time Machine BU

How can I limit space taken by Time Machine BU files on my MyCloud Ex2Ultra? The slider in the MacBU tab moves, I press ‘save’ but it resets to max when I return to it. In any case, that control only works - if it works at all - on the gross space taken by all BUs. Surely it is fundamental to the concept of a NAS that each user’s space - including that for BU - should be controlled, otherwise the TM BUs will consume the whole drive.
I asked WD Support this question but received a very unhelpful shrug from what seemed to be a non-technician.
I’m already using MC5 btw.

Hi @Greigor,

Please ensure the share is set to the default TimeMachine Backup share.

And also ensure the Time Machine Backup Maximum Size is set to the default of 100% for the 1st backup and high enough for subsequent backups.

Western Digital is committed to providing our customers the best possible service and support. As such, we wanted to remind customers that Time Machine is an Apple product and the best place to get support for using Time Machine is with Apple.