Years ago, when I first started using a HE-tunnel, it didn't take long before I disabled IPv6 on my wireless network because Android (v5 on a Galaxy S4 back then) was being a pain in the butt. I don't remember what the problems were exactly, only that when IPv6 became active there were problems.
The problem was the WiFi driver. Samsung for some reason thought it should block ICMPv6/NDP in standby to safe battery. That means the IPv6 connection would go stale. However, the handset wasn't informed about this and tried to continue using the stale IPv6 connection, until it would eventually timeout and fallback to IPv4.
I had the same handset, and those problems are still prevalent today when I connect it to my IPv6-only WiFi. Even worse: it's almost impossible to update anything because as soon as the screen goes off, the device loses internet connectivity. And for some other reason 464XLAT doesn't work on WiFi connections on the Galaxy S4.
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Not too long ago I switched from a /64 to a /48, giving each VLAN it's own /64, this is when I decided to give IPv6 on my wireless network and current phone (Android 8, Galaxy S7) another try ..... and again problems.
The internet connection on my phone varies from working normally, to longer load times, the occasional timeout and sometimes stuck on loading. A restart of the apps in question usually solves that, temporarily.
I now use a NoteFE, which is equivalent to the S7 (except for the SPen and it got upgraded to Android 9). Not problems with IPv6 anymore. However, RAM management has gotten worse with some update, and since then apps are frequently sent to sleep in the background.
There could also be a compatibility issue with your WiFi access point? Which one do you use? It seems that in the past they often didn't send enough "keep alive" (forgot the correct IPv6 terminology). Maybe it's got to do with this?
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So... does anyone have any idea's about how to troubleshoot this? From the limited amount of testing I could do, it looks (to me) it might be an intermittent MTU issue. Normally that indicates firewall issues, but if it was nftables on my router-pc blocking it, all devices would've been affected. Plus, the router-pc's firewall is set to allow all ICMPv6 packets.
MTU often causes weird failures. Maybe try this site to determine you max MTU?
http://www.letmecheck.it/mtu-test.php