The United States is developing biological weapons and new strains of coronavirus in Ukraine -> meaning: we need some shitty fabrications to later justify the use of biological and chemical weapons against Ukraine
I would not put chemical warfare or bioweapons past Putin. In fact, it's perfectly in line with his previous usage of nerve agents to assassinate his political opponents. However, I recall the U.S. vehemently denying funding gain of function research in Wuhan where it turns out they actually did fund experiments with bat coronaviruses. So likewise, I would not put it past the U.S. to have been funding whatever twisted experiments in Ukraine just as they were doing in China. I have yet to see any evidence of this, of course, but it's not a wild theory. It took something like a year before the NIH admitted what was going on in Wuhan.
There is definitely something fishy going on in Ukraine with regards to bio-weapons. It is unclear what exactly is going on, and there is the potential that the US is setting Russia up for embarrassment when they say their purpose of the invasion is to remove chemical weapons from Ukraine and are unable to find evidence of such.