It's more common than you think to see say 12 losses in a row in practice, than seeing 12 wins. Hell you rarely see five wins in a row.
If you're playing with 50% chances, you will see 12 wins just as often as 12 losses so I'm not seeing where that comes from. Even at 49.5% it's going to be pretty similar unless you're doing hundreds of millions of repetitions.
You would be surprised. On PrimeDice I've seen 12 consecutive losses so many times, but never seen more than 7 consecutive wins and that's with a 1% house edge.
But Martingale is definitely a loser in the long term, unless you're super patient maybe.
How often you see each short-term will depend on how the site handles them:
If it is 50/50 with 1.98x return, you should see both equally
If it is 49.5/50.5 with 2x return, you'll see one more than the other
You just happened to be extremely unlucky.
That's the thing with Martingale though, all your losses could come in a row which is what rules you out from recovery. I think that Martingale works well when used intelligently, with discipline.