Unzipping and re-zipping allowed it to get by the online sideloading, but nothing happens when I try to install it manually on the playdate. It'll say it installs, but I can back out and check again and it will go back to showing as uninstalled.
I fixed it! The power shell script that compressed the build used a compression algorithm not recognized by the play date website, so instead of having a script automatically compress the build, I do it manually every commit :)