EDIT: Looked a little deeper/better on GitHub and found this issue, #4865 which is likely the most related issue, and it seems the devs are aware.

It also seems to be a recent v19.5 -ish issue too from some of the comments there

I seem to be encountering what may be a bug with pinning/featuring posts ... interested if anyone's got similar/counter experiences

The issue is that the pinning of a post doesn't get federated correctly.

The conditions, AFAICT are:

• Post originates from a "federated instance" (IE, an instance other than the community's home instance)
• The mod action of pinning is also done by a moderator on a "federated instance"
• Lemmy versions 19.4 or greater (much more tentative, but from a brief perusal, it seems true)

The effect seems to be:

• The pinning works fine on the "home instance" of the community
• But federation breaks in two slightly different ways:
  • No pinning occurs
  • If a mod on a "federated instance" tries to pin, after an initially failed federation of "pinning", it will succeed on the federated instance only temporarily

The last dynamic is hopefully a clue to what could be happening (sounds like some queued tasks colliding in an incorrect way)