You crash an end-of-life trainer dressed up as an F35 in a carefully chosen site with no natural foot traffic. The FBI then sits in a bush and arrests everyone that turns up.

The F35 is the prime intelligence target for Russia before it gets in Ukrainian hands so they might risk some of their most valuable "illegals" i.e. the long-term embedded agents with flawless identities. For a big target, you gotta go big. Hang out the biggest prize in the most embarrassing way you can. This is a typical type of plot from The Americans TV show.

That or it's testing the adversarial satellite or balloon surveillance tech for how long it takes to find the craft. Maybe it's a test of Open Source Intelligence networks. Maybe it had genuine stealth tech and they are monitoring channels to find out if it was being successfully tracked. A top of the line stealth craft is hell of a honeypot so there are endless things that might get revealed - I imagine all hostile intelligence networks are lit up like christmas trees - it's like a barium meal.

Do I believe any of these? Nah. Reality is more ridiculous than fiction so no doubt it really is the case that they have just lost a prize intelligence asset.