At the 2018 Libertarian National Convention, delegates were treated to an avowed communist standing on the debate stage, running for Chair of the National Committee. To many of us, this was an outrage. Meanwhile, the communists themselves, of which there were a sizeable number (about 50 to 60 out of a packed hall of nearly 900), insisted that their assertion that "rent is theft" did not mean they were going to seize landlords' property outside their little voluntary commune. As such, they insisted, communism and the Libertarian Party were perfectly compatible.
There are several issues with that.
First is the assertion that "rent is theft", which stemmed from the assertion that "property is theft". This reasoning is completely incompatible with the foundations of the Libertarian Party's propertarian stances, which, along the lines of Murray Rothbard (one of the founding members of the party), view property as an extension of the self. Communists purport to believe in self-ownership, but claim that property rights are not inherent whatsoever.
Setting aside all of the inconsistencies and leaps of illogic necessary to assert that property is theft, such a belief is not compatible with "propertarian libertarianism". The Non-Aggression Principle (NAP), which the communists also purport to subscribe to, certainly permits one to intervene on behalf of another whose NAP is being violated. If that is the case, a landlord renting to someone else outside the commune would be viewed as aggression, and such aggression could be intervened in. In that case, communists invading and occupying neighboring properties which did not subscribe to the same belief that rent is theft would be ethically permissible. This means that it is not compatible with propertarian libertarianism at all.
In debates I have had with communists, they have avoided this by asserting that you do not have any right to intervene on behalf of someone else whose rights are being violated. If that is the case, the entire belief in the NAP at all falls to pieces. Ethics and morality must be universally applicable. You must have a single standard which is assumed to apply to all human beings. Libertarians most certainly would think that, for example, homosexuals in oppressive Islamic regimes are having their rights violated, and they most certainly would think that under normal circumstances, interfering on behalf of that person, even as a foreigner, is ethically permissible, even if it might not be the most intelligent thing to do (because you yourself would probably not escape unscathed). It doesn't matter that that Islamic country is not a voluntary society (since it is, after all, a country with a government).
That aside, if property is theft, any voluntary propertarian society which formed independent of your commune would be established on property lines which would be strictly enforced. This would be considered theft by a communist, and this once more the grounds would be present to invade and occupy that neighboring propertarian society, since its mere existence would be considered aggression.
On the other hand, while anti-propertarian libertarianism does not permit propertarianism in any way, shape or form to exist in the world, propertarian libertarianism permits any form of libertarianism, including something resembling anti-propertarianism, to exist. Those who detest rent and propose instituting communism can go out, acquire their own plot of land, and live in their communist utopia upon that land. But they would not have the ethical grounds to go out and invade neighboring lands and make them communist.
Simple assurances by communists that they would not invade and occupy propertarian societies is not good enough. They must categorically reject the entire belief system which opposes property. Not only does it not make sense, but because of what it would permit, it is entirely incompatible with the proudly propertarian Libertarian Party. Of course, it makes sense that they would try to infiltrate and seize control of the Libertarian Party: since they do not believe in property, it makes a lot of sense that they would apply the same standard to the Libertarian Party as they would to a propertarian voluntary society: something which must be seized.