Timatic, the database used by airlines to check documentation requirements, currently states:

Warning:

Visitors not holding return/onward tickets could be refused entry.

So VietJet Air are just doing their job.

I can’t tell you if that policy has changed recently or not (possibly in relation with COVID policies), or if its actual application has changed, or if VietJet Air are just more risk-averse than other airlines, or maybe they have faced refusals for this reason in the past (for Japan or for any other country) and now want to avoid any issues, but IMHO any airline which wouldn’t check this would be taking a risk.

Timatic, the database used by airlines to check documentation requirements, currently states for entry into Japan (here, for a UK national, without indication of residence) but this is probably more general):

Warning:

Visitors not holding return/onward tickets could be refused entry.

So VietJet Air are just doing their job.

I can’t tell you if that policy has changed recently or not (possibly in relation with COVID policies), or if its actual application has changed, or if VietJet Air are just more risk-averse than other airlines, or maybe they have faced refusals for this reason in the past (for Japan or for any other country) and now want to avoid any issues, but IMHO any airline which wouldn’t check this would be taking a risk.