>> basic handling of sample modes in the archiver appliance is basically the same as with the older channel archiver.
This is correct.
There is also question 4 in the FAQ that has some more details -
https://slacmshankar.github.io/epicsarchiver_docs/faq.html
You can also look at "camonitor -m al X2:IOC:-FM1_INMON" and see which samples make it into the archiver. This can sometimes help you figure out where the issue is.