Is your assumption that the Russians do not know that "tanks require infantry support" or just ignore this fact?
More likely lack of training, discipline and morale.
The Russian armed forces are still prone to inter branch fighting and intra branch infighting between officers.
Basically they still operate like the military of some el presidente banana republic.
And currently there is no larger than life actually competent field marshal Zhukov like leadership to make all of them fall in line.
Many of the videos from Ukraine have been ambushes (sort of a separate matter) or of AFVs operating without close infantry support. That's not a good way to do things.