For that reason I didn't bother setting my zone to handle only my small subnet's reverse mapping, since it is actually a bit complicated from what I saw, involving a practical trick of using CNAME aliases, as the minimum de-facto supported arpa zone is a C class.
Then what have you done here?

and request the authorative servers for this C class (belonging to servermatrix/theplanet) to delegate authority for my subnet at my DNS server.
I think that the two statements above may be conflicting, (though I might not be totally comprehending the problem). It seems to me that you "requested" the the authority for your 8(?) IP's be passed from theplanet.com top your name servers. Then you _seem_ to be saying that, because of an issue of complexity, you are really trying to be the authority for the entire zone for the C Class, (but that's per the RFC, "minimum de facto supported arpa zone").

The result would appear to be that you have wrestled authority from theplanet.com for the reverse DNS for the entire zone but then, by the use of a cname, you have messed up the entire zone.

While the answer to the problem may be of interest it would seem that you are affecting others within the C Class. Since you only have 5 addresses I would consider returning authority of the reverse zone to theplanet.com and have them set up your 5 RDNS entries and have done with it......

If you are using windows for the DNS servers can you see anything odd in the DNS Event log, (or anywhere else if you are using *nix)?