It's not unusual for studios to work on multiple games simultaneously, especially they all belong to the same franchise. Although most devote their resources towards one game, some studios assign a handful to start working on a second title. This way, once the first title is released, the rest of the development team can move on to working on the second game, which could either be a sequel, prequel, and/or spin-off.
Having said that, it appears that 343 Industries is working on at least one more Halo game outside of Halo Infinite.
343 is already preparing for a future beyond Halo Infinite
It's crazy to think that 343 Industries has been making Halo games for over a decade. The first-party Microsoft studio took over the helm from Bungie starting with 2011's Halo: Combat Evolved Anniversary and has been making Halo games exclusively since. However, 343 has yet to step out of Bungie's shadow, with many long-time Halo fans still considering Bungie's Halo titles as the best in the series. Although this is up for debate, what can't be denied is how committed 343 is to making each Halo game they develop into the best it could be.
