This issue can be found in two ways.

Firstly, 

When more than 10 commands are run in the same tick to add ticking areas, all of the commands will succeed. This does actually result in more than 10 ticking areas in existence at the same time, as exemplified in the attached screenshots.

If 10 or more ticking areas are currently in existence when a command to add more is run, then all of them will properly fail. 

Secondly,

When creating ticking areas in the same tick, all of the command blocks display the same log:

[[currently existing ticking areas] + 1]/10 ticking areas in use.

As opposed to the expected message:

[[currently existing ticking areas] + [ticking areas being created this tick]]/10 ticking areas in use.

In addition, when multiple ticking areas are removed in the same tick, the same log occurs with a -1 instead of the number of areas being removed.