I recently tried aarch64-current on an RPi3B+ (too many qualifiers - the latest, gigabit RPi), but the whole system hung while CVSing pkgsrc. This was from, I believe, May 5th, 2019, or the 6th.Not tried NetBSD-8, but I recently reported timing issues on NetBSD-current 64-bit running on RPI3B+. During pkgsrc builds, a process would freeze and running "time" command would show the current time was stuck still. Maybe that is what you're seeing.
That's not it - I've seen that, too - but it's entirely possible they're related.
John