I will define a "naturally" tiered map as a map at the tier of which it first
exists in a region on the atlas, for example: Beach is on the atlas (with no watchstone modification) at tier 1. Therefore, Beach is a natural tier 1. Burial Chambers first shows up on the atlas (with 1 watchstone modification in Valdo's Rest) at tier 7. Therefore, Burial Chambers is a natural tier 7.
When using Horizon Orbs on maps of a given tier, it will only reroll within the naturally occurring maps of its tier. In the case of tier 7s, no matter how many tier 7s exist on YOUR atlas, you will always either roll a Burial Chambers or Precinct map because they are the only naturally occurring tier 7 maps.
Steps to reproduce:
1. Play the game and get some watchstones, maps of different tiers, and Horizon Orbs
2. Level up a region on your atlas to boost the map tier of some specific map (your choice as long as the chosen tier has maps that do not naturally occur in it)
3. Use Horizon Orbs on a map of a chosen tier to try and reroll into a non-naturally occurring map of the same tier
4. Eventually use up all of your Horizon Orbs because it is impossible
From the wiki: https://pathofexile.gamepedia.com/Orb_of_Horizons
This interaction was once listed as a bug in official known bug list (originally wording: Orb of Horizon and Harbinger Orbs can only reroll maps into specific maps. ) but since version 3.10.0 the bug is removed from the list. Thus, it is not certain that the orb now only reroll to specific maps is an intended behaviour or not.
Thanks for your report, this is currently intended. Using Orb of Horizons on a map will result in maps which have an identical base tier. The base tier of a map is when the map first occurs on your Atlas.
Pier map can be Tiers 1/4/8/10/14, however a Orb of Horizon will never be able to roll any of the tiers except for Tier 1 as it is the base tier, so rolling a Tier 4/8/10/14 map with an Orb of Horizon will never result in Pier.