Spansh really is the only source of 'all the data' we have now (EDDB long since stopped recording new bodies).
Unless I'm wrong, EDAstro's data (see
here) does the same, so it's there too. The EDSM data backup dumps there are weekly though, not monthly.
On to the main point of discussion then: sure, I've seen it happen, although I don't have any numbers on how many times it might come up, so this is just an anecdote. Bear in mind though that these delayed sending features were added long before fleet carriers though, so rushing back to sell data was a much longer affair then.
Then there's also that there are (or at the very least were) some people watching the Codex for new Green Gas Giants, and rushing to new entries with the aim of getting their tags on them.
This can't be circumvented by delaying upload, of course, but it is much easier to watch EDDN for new GGGs than it is to watch the Codex. The former is automated, the latter isn't. However, with carriers being all over the galaxy, the discoverer does have a significant head start over whoever might be racing to nab the tag.
There's also that that when mapping was added, there were people who went after uploaded ELWs, to tag them en masse. I've had quite a lot of mine mapped by the same two (or three?) other CMDRs. I imagine this is only an issue whenever a new tag is introduced, or when a body type suddenly becomes of more interest than before. Such as thin water atmospheres did with Odyssey's release.
Of course, there's no good solution to this. The only way is to rush out to your discoveries to tag them first, if they are ones you care about.