I have been watching this thread as I am having the same issue.
I am using iOS and then connecting to an Ubuntu 20.4 host which routes a couple of internal subnets essentially.
I had issues at the end of last year where I used 10GB in one day, the only thing that I have connect at the other end is Home Assistant with literally a single toggle option so I doubt that is using much data at all.
I saw this thread, updated all my devices and tried again. On the 1st of Feb I chewed through 8GB of data, and my phone got really hot. I just checked and at the time of the issue I was using iOS version 1.2.8 (which was the latest at the time).
I rebooted my device, turned Tailscale off and haven’t had any further issues. It definitely feels like Tailscale is the issue here, I have very similar usage habits with TailScale turned off and don’t have data issues. It also seems random, like the second time I had the issue, it had been around a month without it using much data at all.
If you think that there has been fixes after the 1st of Feb (NZT) I can give it another go, but my issue is that it runs through so much data in a short space of time it hard to catch and should a few hours go by I can lose GB’s worth of mobile data.
Happy to work with anyone at Tailscale to investigate if they want to touch base with me. I really like the product so I am keen to try and assist in resolving the problem.