Should probably be using a double instead to fix this.
To test, use the following command
/worldborder set 59999950 5
which rounds 59999950 up to 59999952 as only multiples of 4 are permitted for values above 2^25.
In addition, since
MC-187668 was marked as a duplicate of this ticket, it should be noted that /worldborder set 59999968 5 and /worldborder set 60000000 5 result in the same world border size for a different reason.
To fix this, I'd recommend making the maximum border size possible 67108864, and move the invisible barrier at 30000000 out to 33554432 to align with this, as there aren't any bugs that can be encountered between 30000000 and 33554432 that can't otherwise be encountered within the current vanilla range. (The world becoming non-solid at 30 million before 1.7.2, and 32 million before beta 1.8, were actually manually coded into the game, and can easily be changed to 33554432 anyway.)
Alternatively, the world border could be moved out to 2,000,000,000 if MC-194878 is fixed.