draft 15 says:
" The format of an Armor Header is that of a key-value pair. A colon (':' 0x38) and a single space (0x20) separate the key and value. OpenPGP should consider improperly formatted Armor Headers to be corruption of the ASCII Armor. "
However, I see improperly formatted headers because of word wrapping. There doesn't appear to be a way to wrap a long line, so I must reject these as badly formatted (not that other implementations do, it seems).
This seems less than optimal.
I spotted this issue a year or two ago and brought it up. At the time, I had suggested that the doc say "Armor Headers SHOULD be no more than the length of the asciiarmor." But there wasn't much enthusiasm for that at the time.
And implementations seem to like long lines, so I imagine the nuisance isn't going away. It's just another limitation of the clearsigned text format; don't use long lines if you value the sig.