It might still be a developmental aircraft. While it’s not uncommon for the Russian air force to deploy test planes to combat zones in order to collect data in a real-world context, it’s a huge setback to a development effort to lose a rare and expensive test plane during combat trials.
I think this is what the west intended with their sporadic support and rules about how aid can be used. A proxy war of attrition.
Good thing Ukranians are tough.