Why this high-pass filter behaves like a low-pass filter for high frequencies, on this ideal LTSpice simulation?
You have an ideal "single pole" operational amplifier.

A single pole amplifier is an amplifier with a first order low-pass characteristic. The pole determines the corner frequency of the pole. For a real opamp it can be found using the gain-bandwidth product, which is the product of open loop gain and the corner-frequency.

Above 100kHz you see the typical characteristic of a low-pass. A gain roll-off of 20dB/decade and a phase shift of -90 degrees.

Since you are using an ideal opamp it should be possible to set the corner (pole) frequency to a higher value, so that it doesn't affect your transfer function.