Ecloud and IPv6

I had an odd issue that is probably specific to me and my device.
I’m on T-Mobile in the U.S. I’ve been running /e/ on my Axon 7 since last year (multiboot with other ROMs) and more recently on my Essential PH-1 (got it so I could play with Pie but that lasted about a week. Ugh!). All is good on both phones.

On my good old LG G3 I multiboot a bunch of ROMs over the years, mostly microG-based. I decided one day to set up a ROM that was both Google and microG free, using only UnifiedNlp. That’s Candy7 Nougat. I figured I’d use my /e/ account for syncing contacts & calendar. In short it’s been running wonderfully.
Wanted to do similar on my Moto G5s Plus (sanders), also a multiboot device. Chose GZOSP Oreo. I could not for the life of me get DAVx5 to work with ecloud. Tried every type of base URL I could think of to no avail. Eventually I used my Google account.
After a few weeks I tried again with no luck. Then I noticed I was also unable to access ecloud via the web, regardless of the browser. Webpage unavailable and all that. Didn’t know why and figured maybe something was wrong with the ROM.

Scrapped it and went with AOSiP Oreo. Used previously as a Google free environment with Simple Mobile Tools as the main apps. Alas, had the exact same issues. DAVx5 wouldn’t work and browsers couldn’t connect to ecloud. A Mozilla browser said to check network connection but there’s nothing wrong with my connection. Wouldn’t hurt to check though.
APN settings are pretty much the same across my ROMs and devices. APN protocol is IPv4/IPv6. Decided to change to good ole reliable IPv4. Lo and behold, that was it.
DAVx5 setup and synced without issue. Browsers brought up ecloud also. Wha? After everything was good to go and working I switched the protocol to just IPv6. Browsers could no longer aceess ecloud. Back to IPv4 and all was good…

What I thought was an unrelated thing, a new download manager from F-Droid, Download Navi, had issues with a number of sites and couldn’t download files. New app so I thought it was just buggy. Switch to IPv4 fixed that also.

Odd that Oreo ROMs on my Moto G5s Plus using IPv6 can’t access ecloud at all but other ROMs (mostly pre-Oreo) on other devices don’t have a problem. At this point I don’t know if it’s an Oreo thing or with the device. I’ll have to boot into each of the other ROMs to confirm.
Any insights on this peculiar issue.