It definitely sounds like you did everything correctly and troubleshooted this in all possible ways. It might be the bootanimation lib that's included in AOSP. You could try replacing the one in /system/lib. I've attached the one from stock Pie. I had to zip it because XDA reasons.
Just tried this, it still doesn't work :/
I asked @Kuntitled on Telegram about it, and he said GSI boot animations work fine on his Aljeter. So then I guess something else is broken with Jeter.