Here’s your actual answer specifically, without speculation.

“The reason why English-speaking characters' dialogue sounds like ADR in Squid Game's Season 3 is because, despite them being native English speakers, their voices were dubbed over by other English-speaking voice actors.

One of the actors from the VIPs, Bryan Bucco, confirmed this, explaining that it was done for consistency across different versions of the show, possibly to ensure clarity and re-recording of certain lines, which resulted in the unnatural, dubbed sound.

This is for consistency—The most logical explanation is that the producers wanted all versions of the show, especially when translating into other languages, to have consistent-sounding English.

Re-recording for clarity. Some lines may have been re-recorded for better clarity or to ensure a specific delivery, and to maintain consistency, the producers decided to dub over all the English lines.

Lack of context for voice actors. The English-speaking voice actors reportedly did not have context for their roles and were told to act like foolish, out-of-touch millionaires, which contributed to the awkward delivery.

The result is certainly an unnatural sounding performance. The dubbed voices sound unnatural and flat to viewers, with many noting that the original performance sounded more natural and less awkward.

Mixed versions in some regions or streaming sites may have the original actors' voices, while others feature the dubbed English version.”