If playing the new snapshot with minecart improvements enabled, minecarts with furnaces no longer can link. To replicate this, open minecraft snapshot 24w33a with "Minecart improvements" enabled. Then, place rails and a furnace minecart with any other minecart variant (that isn't a furnace minecart) right click the furnace minecart to press into the other minecart, and then right click the furnace minecart in the opposite direction. Usually, the minecarts would couple, but instead the furnace minecart just goes without linking to the other minecart.

My best guess would be that since minecart stacking was removed, it cannot link.

If you are looking at this, it would be nice for furnace minecarts to be fixed altogether since they are already broken with the linking feature in any version. Please look into that too.