When trying to load a single chunk with /forceload a 3x3 area of chunks is made accessible instead.

I would expect /forceload to load exactly the chunks specified, no more or less.

It might be that this extra chunk is meant to make the chunks specified in the command entity processing chunks.
However to achieve that a 5x5 area would need to be loaded around each forceloaded chunk or the condition for entity processing chunks would need to change (it might be that the 5x5 is no longer needed due to recent world generation changes).

Either way, in my opinion forceload should only load the chunks specified in the command. It is always possible to load a larger area for the user for the command if an entity processing chunk is needed.
_{Learning a bit more about the new chunk loading mechanics, the above paragraph is nonsense.}
It however would be useful if loading chunks as entity ticking would be an option for the forceload command.

I made a comparison video of chunk loading in 1.13.2 and 18w50a where this is also visible: https://www.youtube.com/watch?v=H3dMoFulIog

The forceloaded chunks are on the lower left side marked with a different color.
Both sides of the visualisation were recorded in the same world with the same forceloaded area. The tool clearly detects one chunk more around the forceloaded area. Manual tests with commands also confirm this.
Good command to test if a chunk is loaded:

/execute unless block <x> <y> <z> air{N:O} run say chunk is loaded