Here’s what’s really going on.
In preparation for a surprise invasion of the Sumy province, RF troops have been gradually clearing mines from the Kursk side of the border. Stealthily,
slowly, at night.
The RF forces have been held back, dozens of km from the front (duh, concentrating them close to the border would tip off the UAF).
Several weeks ago, UAF figured this out. They equally stealthily assembled their own strike forces, and launched a well-executed pre-emptive attack. Russian conscripts and border guards were overwhelmed.
Now the UA brigades find themselves stranded deep in Russian territory. They sliced through the low ground like machetes through butter. But further advances will be uphill, where the Russians have a 100-150 m elevation advantage.
Great short term PR move for Z. But the next few weeks will be another futile meat grinder as the Kursk forces get pounded by air strikes, while his troops in the Donbas will continue their steady westward retreat.