I don’t work for Asana and I’m not necessarily defending this feature, but it is a feature, not a bug (a bug is when software doesn’t work as designed, and in this case, it is working as Asana designed it).
It’s not a technical matter - as you say, technically it can be allowed - but rather Asana built it this way for privacy. As explained to me, they consider one’s My Tasks as an area where a user should be able to “do things privately” without worry about other users seeing it. Again, not necessarily defending it, just explaining.
One thing I will note: since they implemented this design, they have recently introduced more access control capabilities on custom fields in general, so maybe that was a first step to changing this My Tasks behavior. Just speculating…