I did some digging, and found that this was bug #231 in the 6.3.0/6.3.1 development cycle. The bug was fixed on July 19, 2002, meaning it went into the 6.3.1 release.
(The bug was reported internally by our private beta testing team, which is why there's no mention of it here on Community.)
The only solution to the bug is upgrading to the latest release.
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Charles Capps
UBB.classic™ Developer
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