When the first power is applied and the SD card is initialized, S18A responds with 1 in response to ACMD41. So it can be converted to 1.8V.
In this case, since the 1.8V interface is used, communication at the high speed of SDR104 is possible.

However, if reset the MCU, it will respond to ACMD41 as
> command.response[0U] = 0xC0FF8000 (24th bit is 0)
So the 1.8V conversion fails and the IO speed of the SD card is limited to 50 MHz.
Probably already at 1.8V, the S18A response seems to be zero.

Is there a way to check if it has already been initialized to 1.8V so that a correct re-initialization is possible?
In Teensy 4.1, SD card power control is not possible, so it must be solved by software.