MKE02Z SPI Communication TxBuffer not copied - arm

Question

I am Trying to Implement an SPI communication between the Freescale FRDM_KE02Z40M(MKE02 MCU) Evaluation-Board and the ESP32S3.
The ESP32S3 sends 32byte of data. The FRDM_KE02Z40M should send 32 byte back.
However, while the data send by the ESP is recieved correctly, only the first byte of the data the FRDM_KE02Z40M should send is actually transmitted. After that, the ESP32 only recieves his own transmission repeated back to him.
Daten send from ESP32S3:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
data MKE02Z (FRDM-Board) should send:
BB BB BB BB BB BB BB BB BB BB BB BB BB BB BB BB
BB BB BB BB BB BB BB BB BB BB BB BB BB BB BB BB
Daten recieved by ESP32S3:
BB 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E
0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E
Data recieved by the MKE02Z:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
The relevant code of the ISR looks like this:
void spi_slave_irq(spi_handle_t *handle){
uint8_t flags = spi_getStatusRegister();
//Check wether the Transmit Buffer empty flag is set and if so, write the next 8 bit to SPI0_D
if(((flags & 0xFFU) & SPI_S_SPTEF_MASK) != 0)
{
spi_write_shiftregister(*(handle->txData));
handle->txData++;
handle->txindex++;
}
//Check wether the Recieve Buffer full flag is set and if so, read the next 8 bit from SPI0_D
if(((flags & 0xFFU) & SPI_S_SPRF_MASK) != 0)
{
*(handle->rxData) = spi_read_shiftregister();
handle->rxData++;
handle->rxindex++;
}
...
The RxBuffer Full Flag is reliantly set during transmission and the shiftregister copied to the rxbuffer every 8 bit.
However, the TxEmpty Statusflag is never set during a Transmission but only BEFORE and AFTER it. When the MCU is reset, the txbuffer empty flag is set, the interrupt called, the buffer filled and directly copied to the shiftregister. The TxBuffer Flag is then raised again, interrupt called again and txbuffer filled with the second byte. The System then idles until communication is initiated by the master. During Transmission (when SS is Low), the TxBufferFull Flag is never raised again. This leads me to the assumption, that the TxBuffer is not copied to the shift register of the SPI Module during the Transmission.
This makes sense, as if the TxBuffer is not copied to the shift register the data recieved from the ESP32 would stay on the shift register and be repeated back to it the next 8 shifts.
I provided a Sketch of how the SPI Module works according to data sheet and reference manual:
SPI_MODULE_Sketch
Is such a Problem known? Could it be that the SPI-Module on this MCU is faulty? (I tried it with 2 different Boards. I used 2 Drivers provided by NXP (Freescale Driver and CMSIS Driver) and wrote my own directly following the Reference Manual. NXP is not reacting to contact requests until now.
The error exists exactly the same within the example projects, my version and on both Boards.