There is no general rule against one company or person buying both broadcasting rights and merchandising rights to a particular piece of content in the US.
There are anti-monopoly/anti-trust laws, but those generally only apply if a particular entity holds a monopoly or a commanding market position in a whole market sector. If one firm held the rights to 80% of all online games, for example, an anti-trust action might well be warranted. But a single game or property is not generally considered to be a market sector for anti-trust purposes. Exactly what the proper market sector is in such cases is often a complex, technical, and highly disputed issue.
The "original owner of the IP" can decide who s/he wishes to sell that IP to -- nothing requires, or forbids, that different sets of rights be sold to the same buyer. The original owner will attempt to get the best deal available. Sometimes that is a very lucrative deal, and sometimes it is far from that. As long as unlawful methods are not used to induce a sale, whatever bargain the parties make is generally acceptable to the law.
I do not know who did, or did not, buy any of the rights to Squid Game, and that info might not be publicly available. But there is no law that I know of against the same party having both broadcast and marketing rights to it, and perhaps other rights as well.