i would check the ciphering mode in both cells.
if both sides are using the same ciphering mode (like A5/1), then it’s something else…
To analyse it, double-check the content of the HO COMMAND received by the MS. Something in this message is irritating the MS.
Frequency ? Ciphering ? Packet capabilities (is this a DTM call ?)
Post the content of the HO CMD here if you want, we can check it together.