disabling data roaming on Nokia 6.1 (Android 10), service provider o2 (UK)
Selecting 2G as preferred network is the wrong decision as AFAIK Japan has no 2G network anymore (has been switched off some years ago). The minimum is 3G.

Because of VoLTE the definition of "data roaming" is become a bit complicated. If your device connects to LTE and has VoLTE enabled it starts a data connection. A lot of providers fail to identify those connections as "Voice" and charge them as data roaming. Therefore VoLTE connection start-up are charged as data roaming. 

Hence it is very important not only to disable data roaming but also disable VoLTE feature!

I also run into this problem on two-day trip abroad where my provider wanted me to charge data roaming for a few bytes every time when I switched on my phone or disabled Air-plane mode. Back from my trip I found a few mobile data roaming charges on my bill even with mobile data roaming disabled (phone has VoLTE capability). After complaining at my MNO they removed them from my bill by goodwill.