I find it somehow difficult to understand why should apn change cause this kind of different behaviour.

If I am not totally mistaken the android subsystem running on a sailfish device relies on the sailfish side for network access, so that if your sailfish applications can access network (rmnet_data0 interface is in UP and RUNNING state) then the android applications also must have network access, it is just pure ip routing issue...

The one difference indeed would be if the android side wants to use different protocol than SFOS side, one desiring IPv4 and other IPv6...
but that would be silly indeed; nowdays you'd expect applications to be network protocol independent.