Actually dead of winter is a good time to invade. Ground is frozen. In spring, Ukraine is famous for its deep mud. Tanks would be confined to the roads.
“In this season you have what we call Rasputitsa, which is mud,” said Pavel Felgenhauer, a Moscow-based military analyst for the Jamestown Foundation. Yet for any attack on eastern Ukraine, he said, Russian generals would want to move fast to outflank the main Ukrainian force around the conflict-ridden Donetsk and Lugansk regions, before its soldiers have time to dig in.
“We have to be able to move our tanks and other vehicles off the roads to do that, which in the black earth region down there would be impossible right now. Once the frosts come you can move in any direction you want,” Felgenhauer said.
https://www.bloomberg.com/news/articles/2021-11-25/mud-could-help-decide-timing-of-any-russia-move-against-ukraine
Exactly.
This “pullback” is a feint. Classic Art of War - appear to be pulling back just before an attack.