After more searching I finally found the answer in this previous thread:
The short of it is that indeed, all of these variables are not automatically saved online, even if you tell the Builder to save them. So, you have to add additional lines in your JS code to make sure that they are.
I’ll leave this thread up to improve findability.
(To clarify, it turns out my issue wasn’t really caused by these components being inside a loop. Stimuli presented outside of a loop will have the same problem. Responses normally do appear to be saved automatically (if you ask Builder to do so), but not in my case as I’m using a less standard way to collect them, not implemented through a simple ‘keyboard’ component.)
I don’t mind that this is the case - we’re lucky that PsychoJS even exists and that people are willing to develop and maintain it for us, and I am grateful to everyone who contributes to this. I’m just wondering: was this explained somewhere and I just missed it? I totally understand that this is a missing feature, but maybe it would be good to just add a brief warning about this in the Builder GUI (e.g. in the ‘data’ tab for the affected components)?