Series1, ApiMode, MacMode==802.15.04wAck: PackageLoss/Problems with FrameID 0x63/RmtATCmds not properly

Hi Community,
I got some XBee Series1, Firmware 10E8, BD == 9600.
Using API Mode. All are configured as EndDevices.

Formerly they were using MacMode==0 (802.15.04 +DigiHeader with Acks):
With that config I observed silently lost TX Data packages approx. 10-25%. These were TestResults of UnitTests with sending 1000 Tx Packages with same Payload, different FrameID. The Tx-Xbee wasn’t reporting any TXStatus.
-Someone had same Issue?

When I configured the XBees to use MacMode==2 nearly no Packages are lost, except those with FrameID == 0x63.
For those there is no TXStatus reported.
-Someone had same Issue?

Now, I try to reconfigure XBees in Devices (XBees are soldered, no way to remove them from Devices) via RmtATCmds.
In User Manual it is stated that when Command Options (byte 16) is == 0 the changes are applied by sending WR and FR or AC.
But I observed that when I send out RmtAtCmd MM==2 it seems to apply directly, which is not given me chance to Issue WR. Means that after PowerDown the Xbee is back in MacMode==0.
-Someone had same Issue?

Ok, as workaround I reconfigured the Tx Xbee to Use MacMode==2 as well after sending out RmtATCmd MM, to send the WR now. But what I observed now is that with having MacMode==2 the RmtATCmds are not working at all.
-Someone had same Issue?

This is really a mess, any Ideas, any Help!?!?
Thanks in advance.

I have checked the Packages with the Packet-Check.tcl as well.