That issue has definitely been reported for Sam460 by ACube to us but has not been fixed in MorphOS 3.20.
I will look into it for MorphOS 3.21.

The PMacG5 related issue is most likely completely unrelated to that, even if it shows a similar behavior in the end.
Using any PCIe card in the PMacG5 that is not graphics card , nor a rtl8168 network card has never been properly tested.