How to understand the "a write to mmpt with an unsupported mode value is not ignored"?

[rock-ifly]

Q1: Are there any comments on the following description on mmpt? What does the "not ignored" mean, report a fault? if so, what kind of the fault, illegal instruction fault?

Implementations are not required to support all defined MODE settings when XLEN=64. A write to mmpt
with an unsupported MODE value is not ignored. Instead, the fields of mmpt are WARL in the normal
way, when so indicated in Register 1 and Register 2.

Q2: The WARL field should keep the original value if unsupported value is written? does WARL mean that if a write to mmpt with an unsupported mode should be ignored, and keep the original value in the mmpt.mode?

RISCV privilledge vol2: Implementations will not raise an exception on writes of unsupported values to a WARL field.