[bestham]

That is the exact same phenomenon. Artificial sharpening is introducing high bandwidth components to a signal. If you bandwidth limit (low pass) a signal to fit below the Shannon-Nyquist limit you will get ringing as the signal cannot be represented accurately and will smear in the time domain. Given a bandwidth constraint, artificial sharpening above a certain threshold will result in ringing.

[astrange]

Images don't have infinite bandwidth, so that doesn't apply. The filter used in H264 and newer codecs is exact and nearly reversible, there aren't artifacts from applying it. The artifacts come from the rounding afterward.