The tank on my 1250 GS looks identical to my previous 2017.5 model year bike. I think there are some minor differences in the inner parts of the fairing and maybe the inside of the radiator covers but externally all the panels look the same as the 2017.5 bike, though this did have some panel differences to the earlier LCs, but I think these were limited to the radiator covers, so it is unlikely that the tank is different.
From the following, it looks like the tank has the same part number for 1200 and 1250:
In that case, although the tank itself is the same, the tank side cover panels are a little different on the earlier models, in particular the bit around and just above the inlet tubes to the cylinders, so this may well affect the fit of the Bagster cover. Probably best to see if you can get an answer on this from Bagster, though having had reason to contact them regarding the poor design of their cover for a different bike, I didn't find them terribly helpful on that occasion.