If you place a beacon on a pyramid of metal blocks, and that beacon has blocks at least 10 blocks above it that its beam can shine through (such as glass), it won't grant advancements to the player who placed it.
- Build a beacon base (with four levels, to test both advancements at once)
- Place a pillar of 12 glass blocks where the beacon would normally go
- Remove all of the 11 glass blocks below the top one
- Place a beacon on top of the beacon base
→ You don't get any advancements
- Break the beacon again
- Remove the remaining glass block and/or place a glass block below it
- Place the beacon again
→ You get the advancements
I just created a fully powered beacon in my survival world in 1.14.2, and the "Beaconator" achievement will not mark complete. I know the beacon is fully powered, as I've been given the status effect of "speed II," which is only possible at the highest tier.