[CAN] Laid off "without cause" officially but told it was because of performance. What should I tell future employers?

Basically I was laid off for reaaons related to performance during my 6 month probation. The role was going to be fully unionized in 6 months and at the 5 month mark they canned me as that was the best time to let me go cause after become unionized it would be difficult. Officially it says I was laid off "without cause". In person I was told because it was a new software I didn't improve upon and they didn't like my rate of progress on it. It is what it is unfortunately.

I am not sure what to say to future employers when they ask. "Without cause" implies it was through no fault of my own but I'm sure they'll ask anyways and itll sound odd that I was let go so suddenly. But then if I say because of performance it'll be a sudden rejection. So I'm not sure what to tell future employers when it comes to this question.

Any tips for this appreciated. Thanks in advance.