This conflict has been going on for several months, with both sides crossing each others border. This is neither new or surprising. Russia has harbored and treated the Ukrainian wounded with no problems. They also made not much of a fuss, of a lost Ukrainian column who drove several Kilometers towards Rostov. They've been very lenient on cross border shelling also.So it's okay as long as it's a limited invasion (and yes, sending your armed forces into another country without their permission is an invasion) is what you are saying?
If the Republic of Georgia wanted to send a small column of armor (another one spotted today) and ADA with artillery support into Abkhazia or South (or even North) Ossetia, or if Azerbaijan wanted to do the same into Dagestan...that there would be no need for Russia to make a "big fuss", as long as it's a "limited" invasion? Since you want to bring equivalency by bringing private security companies hired for internal security vice a nation-state's national military forces into the equation, it would also be fine if either of those two nations hired Academi to do the same (eg stage a limited incursion into another country)?
Or if the PLA wanted to do the same out in the Far East...again, fine as long as it's a limited force?
Frankly you see these instances a lot; just a few days ago, PLA soldiers crossed into disputed territory with India. Though, that's a completely different situation than Ukraine.