I'm pretty sure you can't get the new cells by random drop unlike last season so if you're interestend in getting them you'll actually have to play gauntlet. Or i was just really unfortunate...External link →
the ones from last Gauntlet were available from cell fusion as a bug previously, but are meant to be obtainable from fusion normally after the 1st season of Gauntlet ended. the new cells shouldn't be able to drop from middleman
This is correct, you should be able to get previous seasons cells this way but not the current season. Being able to previously was a bug.
the newly added cells stays forever? I thought it would be removed when the season ends.
Yes, they stay forever. They go from exclusive to gauntlet during the season to having to get them through The Middleman after the season. This can take a lot of dust and/or time through, so it's much easier to get them during the season if you play gauntlet.