generating mazes using Eller's algorithm - why do I get loops?
here I get two blocks of the same set, but they don't go one after the other. So I join the second one go it's own unique set


Don't do that; this is not a step mentioned in your first reference! Two cells are in the same set when there is already a path between them (by going up though the already generated portion). By arbitrarily placing the second block into a new set, you are ignoring the fact that it is connected to other cells.

When you leave the 3 blocks alone, you will get

(03 17)03
 03(03 03)


You are required to place a wall between two blocks of the same set, because they already have a path between them. Thus you produce

 03 03|03


And don't had a loop.