Looks like Russia's ground forces have decisively changed tactics over the past 10 days or so.
Gone are the rapid thrusts down single roads to capture population centers from which they could then consolidate territory.
Russia is now applying general pressure across a large area of front and then deploying several reinforced Battalion Tactical Groups (BTGs) to undertake mutually supporting, careful advances in key areas - supported heavily by tube artillery and tactical air power. These BTGs will advance against Ukranian opposition where they can. Sometimes successfully, sometimes not. Then they will be rotated out of the line to for rest and logistics support. Whatever ground they manage to take will be held by other BTGs. Then another couple of BTGs will take up the advance, in a sort of leapfrog style.
So far its been somewhat successful. Russia has advanced south and west from Izyum, south from Lyman and north from Donetsk City.
However, it's also very expensive in terms of matériel and manpower. Not only is Russia heavily expending artillery ammunition, but its also putting a LOT of armoured vehicles into contact with Ukrainian forces. In the last three days, Ukraine claims to have destroyed 55 tanks, 80 IFVs/APCs, 15 tube artillery systems and three missile systems, three aircraft, one helicopter and about a dozen UAVs, as well as about 900 troops killed.*
Of course, this sort of broader front warfare is also expensive for the Ukrainians. But, they've got a defensive advantage in a relatively static tactical situation, so I suspect the exchange rate is heavily in their favour.
To me, Russia's tactics are reminiscent of Britain's 'bite-and-hold' tactics from WW1: Gain local manpower/matériel superiority, advance with heavy support from artillery, take and hold tactical/local objectives and then repeat ad nauseaum. You don't overextend your lines and wander into an ambush, you don't outrun your logistical support, you don't have to worry about rear/flank security (too much) and you continue to pour pressure on an enemy that is overmatched in terms of total strength.
My worry for the Ukrainians is that all these local/tactical successes are going to add up to a major strategic success for Russia at some point. If Russian BTGs continue to advance west from Izyum or north from Donetsk City, there is a real risk they will outflank Ukraine's defences and heavy forces and find themselves facing much more limited opposition. They can then turn south or northwest and channel their BTG reserves into a proper encirclement. This would force Ukraine to redeploy forces away from the line of contact in the west - potentially giving large amounts of ground to do so - or to pull back entirely and abandon Donetsk Oblast.
*To put that into context, the worst year for coalition personnel in Afghanistan was 2010, when 710 people were killed.