For CSME 11, the "Temporary Disable mode" is accomplished by enabling the High Assurance Program (HAP) bit at the Flash Descriptor of the SPI chip. I assume that's what the BIOS did. Theoretically the FD should be read/write locked so maybe that's why it does not revert back. I wonder how it managed to change it in the first place. If it is an official option found at the BIOS then you should ask Dell how to re-enable it again. Otherwise, you'll need to re-flash the FD region of the SPI chip in order to reverse that set bit.

Have you tried restoring the default/stock BIOS options? That is easy enough and could help in case the BIOS causes the disablement in a different way at every boot (maybe by sending a special message instead).