Mouse fever is a treatable disease with proper medication (which the Russians won't provide to their own soldiers). Meanwhile, there is no cure for hanta virus and the fatality rate is up to 38 percent. If it is the latter, then the Kremlin's war plans aren't worth the paper on which they were drawn.

Russia is said to have lost 87% of its troops, probably this is not all due to enemy action.

The Russian troops have been treated miserably, including being shot if they don't follow orders
or try to retreat from certain death. It is also becoming a very unpopular war.
These factors makes fragging more likely.

As early as a year ago, some time before Pregozhin's mutiny. There were reports back then but nobody knows how frequently that happens.