I promoted 3 minor leaguers who had a full year of experience in the minors before promoting... none of them got an increase in attributes when promoting them to the majors.

I promoted them during the arbitration signing period, was this a mistake? Should i have waited till spring training?

Promotion bumps are never guaranteed.

I was under the impression that "promotion bumps" were a false notion altogether.

The overwhelming opinion was that what looked like a promotion bump, was merely a "snapshot in time", in other words, the same increases you would have seen at the next regular season "snapshot in time"

Has anyone ever actually seen an increase from nothing more than a promotion, (A promotion made before ST, where playing time could not be a factor)

Yeah, I believe they are pretty much just a snapshot in time, and you only get a "Promotion" event when you promote a guy a certain period of time after whatever the last checkmark was.

For instance, I had a guy this year I promoted right before the end of spring training. He's been done developing for a while, but I got a "Promotion" event on his card that didn't show any development, and then he didn't get the "Opening Day" checkpoint that everyone else got. Same for a guy I traded for right at the end of spring training - he was still developing so I got a "Trade" checkpoint and he showed some increases, but again, didn't get the opening day one.

So, to the OPs question, you didn't get a development checkpoint because it was so close to the start of the season. Had you waited until some time during spring training, you probably would have gotten a promotion checkpoint that would have showed development - but I don't believe that would have been "extra" development.

I remember it being put this way.  Players are always either progressing, stagnating or regressing.  The events (opening day, spring training, promotion, trade, etc) just take a snapshot of the players current abilities.  That snapshot will stay the same until the next snapshot is taken, but behind the scenes that player is still progressing, stagnating or regressing every cycle and on and on.