Hi FireDoor!
Welcome to the forum! Hope you enjoy all the stuff available here.
As you mention, the rules do not prevent you from placing an ingot on an adjacent tile with all their features completed. This limitation may be in our heads at first but think that a tile with several features (completed or not) may be involved in the scoring of another feature, for example a monastery or a castle.
This said, you may be interested in placing an ingot on a tile with completed features or with features that no one (even you) can score (impossible to complete or unoccupied) to prevent other players from collecting ingots. Remember that the more ingots you have, the more points they are worth. If you cannot benefit from the ingots, you are allowed to deny this benefit to others.
So the context and the intention are important and the rules give players a lot of room to explore all these options.
Besides these considerations about benefitting yourself versus hindering others, I'd like to elaborate a bit about the possibilities at hand with the following scenario:


draws a goldmine tile that would fill the gap and a number of options will be available depending on his intentions besides scoring the city just completed, since a monastery and a city are also completed:

In this case, you can see that the first ingot is placed on a tile where all the features were just completed, so you can see this is not a limitation either. This shouldn't be a surprise.
>> Scoring the monastery overlapping the completed cityIf

would like to score the monastery, he could place the first ingot on the monastery tile (mandatory in any case) and the second on any adjacent tile he may choose:
a)

may choose to place the second ingot on a tile with incomplete features (a city in this case).

b)

may choose to place the second ingot on a tile with a feature that was just completed (the road in this case).

c)

may choose to place the second ingot on a tile with a feature completed on a previous turn (a road in this case).

So as you can see in these three cases, the monastery overlapping the city will allow

to collect ingots no matter the features on those tiles. After placing the ingots

place one of his meeples on the monastery and will score the features as usual and get the two ingots just placed.
>> Scoring the road and the completed cityIf

would like to score the road,

will collect the first ingot for sure but the second ingot may stay on the board depending on the location chosen for it.
a)

may choose to place the second ingot on a tile with incomplete features (a city in this case).

The second ingot is not on a tile of the completed city or the completed road, so after placing the meeple on the road,

will score both features but will only collect the ingot on the tile just placed. The second ingot could be collected by any player completing the city on the tile or an adjacent monastery, for example.
b)

may choose to place the second ingot on a tile with a feature that was just completed (the road in this case).

The second ingot is on a tile of the completed road, so after placing the meeple on the road,

will score the city and the road. In this case,

will also collect the two ingots.
c)

may choose to place the second ingot on a tile with a feature completed on a previous turn (a road in this case).

The second ingot is not on a tile of the completed city or the completed road, so after placing the meeple on the road,

will score both features but will only collect the ingot on the tile just placed.
Unless an adjacent monastery is completed, for example, the second ingot will stay on the board until the end of the game.
Hope this helps.

Cheers!