The translation keys used for /worldborder set
when the size is too small (less than 1) or too big (more than 60000000) have a period at the end: commands.worldborder.set.failed.small.
and commands.worldborder.set.failed.big.
(for the internal translation keys). This does not match the pattern used by any other keys (keys with a dot at the end are in some places other, but they always have additional text concatenated afterwards), although it has no in-game effect.
Fixed in 21w17a.