OK, very curious indeed.
Yes, I'm running W3 Total Cache, but cleared it before and after applying IP range block (then I tried with turning it off).
Then I tried 198.27.99.0-198.27.99.255 (with no preceding or succeeding space; and no space between the dash). That works.
Historically, I've replicated the IP range I want to block using the example beneath the block fields. The example sets off the hyphen with a space on either side. Stripping that space resolves the issue.
I suggest either allowing a single whitespace on either side of the hyphen or setting the example without the single whitespace on either side of the hyphen.
Thanks for the responses, much appreciated.