There is no other invading army in Belarus, certainly not one capable of doing much. They haven't mobilised, they have 4,500 active troops on their border (the whole border) and about 3,500 in reserve. There are also 9,000 Russian troops there
The Ukraine / Belarus border is now double fenced with minefields. Belarus has given Russia an awful lot of its spare mechanised armour. Ukriane have a number of battalions of troops dedicated to defending this border
There are far less troops and supplies in Belarus now than there were in February, Ukraine is far better prepared and armed than it was then and you can't just walk over the border any more
Ukraine isn't scared of what can come across the ground from Belarus, not only that but there are huge doubts as to whether the Belarus army would actually invade. So far they've refused and those 9,000 Russian troops are more likely there to quell any rebellion than invade. Just like they did a few years back during the last attempt to overthrow Lukashenko when he stole the last election.
But yes, I also doubt the Russian narrative. Absolutely smells of false flag.