Slight play/jiggle in rear wheel, bolt speed-thru axle is fully tightened?

Bike is a Cannondale Topstone Ultegra RX2. Rear hub is a Formula RXC-400, 12×142 centerlock.

I got a flat rear tire on a ride and swapped the tube. When I retightened the thru axle I noticed that there was definitely some play in the rear wheel, even when the axle was fully tightened. Seems like the hub is slightly loose or something. I can provide pictures of the hub body etc if needed. I’m not quite sure how to fix this problem.

Removing the wheel at home and inspecting the cassette, it feels very slightly loose with some horizontal play. I guess I need a lockring to tighten it?

Images available here: https://imgur.com/a/xflq8qL