If I use TPM1_CH0 to multiplex LPUART0 Tx it works as expected:
When I did the same on UART1 with FTM2_CH0 there was no effect when SIM_SOPT5 has 0x00000020 set.
However, by chance, I found that there is an effect but it depends on "both" FTM2_CH0 AND FTM2_CH1!
The modulation on UART1 Tx is only present when FTM2_CH0 is low AND FTM2_CH1 is high!
Since I was also using FTM2_CH1 for another reason and it was low it was stopping output (doing the same with TPM1_CH1 didn't do the same - it had no effect).
(If I change the polarity of FTM2_CH1 it doesn't change the output - still does the same).
Since I need FTM2 CH1 (and it is longer than FTM2_CH0) this therefore doesn't allow the operation that is needed (it stops the modulation working).
Is this effect known or expected? Is it an error in the chip?