This looks like a case of the booking class lists used by BAEC not having been updated to take into account that AY is now selling premium economy tickets, albeit on flights operated by other airlines.

That would mean that - as Prospero says - Avios and TP earning are being reported on the basis that T class falls into the "economy low" category, ie the middle bracket of economy class earning. You can check this by going to https://www.britishairways.com/trave...r/public/en_gb and looking at the earning categories for a Finnair flight on your route. You will see T class listed as "Economy low".

The only assurance that you have is that you will be awarded Avios and TPs accordingly. However, when this kind of lag has happened in the past, ISTR that BAEC has been willing (on post-flight request) to correct the earning to reflect that you actually flew in premium economy. But in your shoes, I am not sure I'd be relying on this unless the precise earning rate doesn't make much difference to you and there is some good reason for booking the AY code rather than the operating airline's code.