Many people say prismatic LiFePO4 cells cannot be put lying down because of the liquid in the cells.
But there are no such requirements in the LiFePO4 cells datasheet, also never heard of such requirements from the cell manufacturer.
Actually, many car battery modules now put cells lying down or even upside down to save space.
CATL CPT3.0 battery pack has upside-down cells.
https://www.tycorun.com/blogs/news/tesla-4680-battery-vs-catl-ctp-battery
SAIC (giant car manufacturer in China) new battery pack has lying cells.
https://www.autoevolution.com/news/rising-auto-will-compete-with-nio-in-battery-pack-swapping-with-catl-s-help-199432.html#
This research paper may explain why any layout is ok in theory.
https://linkinghub.elsevier.com/retrieve/pii/S0378775318300818