Got to wonder if something like these can't be shonked into various sealed batteries:
https://www.flowsystemsusa.com/battery-caps.html
Making sure they were good and tight for 100% sealing, could be the catch. Electrolyte loves to creep.
I've been trawling YT and images, can't find an objective teardown of a sealed car battery. They're either rednecks or boffins.