I don't really think this tracks. The production committee gets final say on the branding and the number of episodes in each contract. The studio doesn't get to play hooky and unilaterally tack on episodes to a fixed contract.

More likely, the production committee underestimated the remaining source material and thought they could wrap things up by commissioning an unusually long 16-episode season. It makes sense why they'd fall into this trap, I feel -- they'd just dumped Wit who had continuously given them scheduling problems and wanted to lock in MAPPA to get the thing finished in a single contract (which they clearly failed to do, since this season has now required 3+ separate production runs)

There's an interview from back in November 2020 which I think demonstrates their eagerness to conclude the series. It's not surprising to me that MAPPA were the only ones to tell them that such a thing would be possible:

Tateishi: "While season 3 was in production, we consulted with Wit Studio, and it was decided that The Final Season would be created by a different production studio. [...] Through many talks with Wit Studio, we understood that the continuation would be difficult, so we looked for a new studio. Kinoshita, Maeda, and I divided the searching duties among ourselves, but most studios turned us down. Everyone understood what big shoes they had to fill, and we were told by dozens of studios that they emphatically could not accept."

Maeda: "Of course, there were also those who told us that they had the desire to do it but just couldn't make it work in their schedule."

Tateishi: "The only studio that said they would consider it was MAPPA. When we talked to WIT about it, they said they could rest easy knowing it was in MAPPA's hands. The producers were in agreement, so the production shifted to MAPPA."

Kinoshita: "MAPPA took the matter firmly into their hands, saying, 'Attack on Titan should have a proper ending for the sake of the fans,' which also made a strong impression on us."

Maeda: "I think you can see their fighting spirit reflected in The Final Season's PV. The show is in good hands."

The producers also stressed that, for the sake of all the fans who have supported the series, they are determined to adapt the manga all the way until the end instead of concluding midway through. Kinoshita said that he has felt obliged to adapt the entirety of the manga ever since seeing the reaction to season 1 episode 13, where the Eren Titan seals the wall.