I have only ever trimmed a stair climb activity once because I forgot to press the stop button until about 15 minutes later. To get a more accurate statistic, I trimmed the activity which worked perfectly EXCEPT it removed all my stair stats. So, although the time was good, it only showed 0 flights climbed when it fact it was >40.
I ended up deleting the trimmed activity, imported it again from my watch, and left it alone. Then I contacted Garmin about a bug. No idea if they fixed it.
My advice is, keep a copy of the untrimmed activity available just in case.
