If this is already known or has been discussed, I apologize.

When looking for award availability on TK, one gets different results if IST is a connection, rather than origin or dest.

This is evident on ExpertFlyer and on TK's own website as well.

Plug in a route to your intended destination with IST as origin. Check award availability. Then set another origin, route it through IST to your intended destination. The award results often change.

They are bookable.