It's very strange. I'm looking at ARN-HEL-BKK in J with stopovers both ways.

Firstly, going via the Swedish site, which is in the new format, choosing multicity sends me back to the old version. Going via the Finnish site, you just use the old version all the way.

Secondly, the website is able to price a ticket where the first and last segments are more than one month apart. The ongoing campaign price, however, requires the first and last segements to be within a month of each other. When trying to book such a ticket, the engine crashes and finds nothing (while ITA does find the price without any problems). So somehow it seems that the booking engine is able to handle non-campaign tickets, but not campaign tickets.