Micro SD card operating voltage: 1.8 V specified?
SDHC and SDXC need to be able to switch to 1.8V IO operation after a certain command, so it's not that surprising that they might also work with 1.8V supply – after all, the switchability most likely implies the internal logic is simply run from a linear voltage regulator that drops the nominal 3.3V to a voltage that is at most 1.8V for all the logic.

Regarding standardization:

https://www.sdcard.org/downloads/pls/

doesn't say that a single 1.8V supply is permissible, so you're definitely driving things out-of-spec (Part 1, section 3.2 "supply voltage).