"Misprison of a felony," I had not heard of such a phrase. Thank you.

The link is from The US National Library of Medicine National Institutes of Health.

In the U.S., to sustain a conviction for misprision of felony it was incumbent upon the government to prove beyond a reasonable doubt four essential elements:

(1) that the principal, had committed and completed the felony alleged;
(2) that the defendant had full knowledge of that fact;
(3) that he failed to notify the authorities; and
(4) that he took affirmative steps to conceal the crime of the principal.

As you pointed out, failure to disclose, without active concealment, is not a felony.

However, you need to "read between the lines" and consider these examples of active concealment:

Giving or making an untruthful statement about issues related to commission of a felony constitutes an affirmative step to conceal the crime:
"No, officer. Gimwinkle never told me about that."

Changing, hiding or destroying official records in order to conceal the fraudulent act:
"Yes officer, I did have a record of what Gimwinkle told me but I threw it away."

Directly or indirectly causing others to withhold or suppress information pertaining to the fraudulent act:
"Okay, Group Therapy Participants, what Gimwinkle just told about himself is just private between us here. No need to share it with your friends."

Making false statements to investigators regarding the fraudulent act;
"No, officer. Gimwinkle only said that he thought about doing it."

Although there seems to be some ambiguity on what exactly satisfies the definition of “active concealment”, it is obvious from these examples, (and subject to the particulars of “concealment” alleged in the query presented), in Canada there is a plethora of codified offences that may fit a particular fact pattern of “active concealment”, examples: a witness refusing to testify may be in contempt of court; concealing evidence or influencing a witness may be subject to an obstruct justice; aiding and abetting or an accessory after the fact.

This seems to be a common theme with "laws". Step across an arbitrary line and a loving and willing sexual partner suddenly becomes a victim of a violent crime and you go to prison for life.