That's the bug I'm referring to here:

This week we deployed the new version to production and we’ve observed one possibly new bug (an occasional notification missing a valid timestamp) that needs further investigation (the dev who is investigating suspects it was actually introduced earlier when changes were made to how hived processes reversible operations, but it is prudent to delay the new release until we’re sure about that).

It started happening after we deployed this experimental version to our api node, and I haven't seen the bug appearing on any other node, so I think it is a problem with the new code that our testing didn't detect. It'll probably take us a couple of days to figure out the issue and fix it. We're leaving the bad code running on our node so that we can analyze it better. In the meantime, if it's bothering you, you should be able to swap to using a different API node.