My experience, from memory; a few of the details might be wrong (e.g. it probably wasn't Redis, but it was some kind of datastore) but the basic experience is what happened:

Q: Why does my webapp run on most of our tomcats but not this one?

A: That tomcat server has the redis client libraries on the classpath, so Spring Boot automatically started up a connection pool connecting to localhost on the default port, and automatically includes that in the healthcheck endpoint. Since redis isn't actually running there, it fails to connect, so the healthcheck always fails and the app never shows as running, even though your code is working fine.

Q: How do I turn that off?

A: Upgrade to the next version of Spring boot and then add this undocumented annotation to your configuration

Q: How was I supposed to figure any of that out?

A: Hahaha fuck you

Spring proper is fine, useful even. Spring boot is an absolute nightmare of COME FROM style incomprehensibility.