Apparently custom app logos that are defined as base64 code or svg code (rather than referencing a file) don't appear everywhere the logo is supposed to display (e.g., svg logo doesn't appear in the app header bar, base64 logo doesn't appear in the app gallery). That should be supported--in effect, it's even a bug that it's possible to enter a custom app logo that appears in some places but not others.
In the absence of support, the constraint should be documented for user reference--e.g., in the Brand: The Essentials article where custom app logos are mentioned or even directly in the AppSheet UI where it's possible to enter a base64 or svg definition for a custom app logo.
LinkedIn
Twitter
