Player Health Indicator Mod 1.21.7 is best treated as an exact compatibility query. The original project currently publishes a 1.21 release and identifies Fabric, Fabric API, and modern Java requirements in its metadata. That does not automatically make it a 1.21.7 build.
The loader is part of compatibility
A correct Minecraft number paired with the wrong loader is still an incompatible setup. This project is Fabric-based, so Forge or NeoForge profiles should not be treated as equivalent.
Dependencies can fail before the renderer ever runs
The project declares Fabric API. If the API is missing or incompatible, the game can fail during initialization long before the heart-rendering code is reached. Fix environment errors before troubleshooting visual placement.
Version matching should be exact
Point releases can update client internals used by rendering mixins. Because PlayerHealthIndicators integrates with player rendering, exact compatibility matters more than it would for a purely data-driven resource pack.
Use a two-stage test
First launch the 1.21.7 Fabric profile with its required API and no health mod. Then add only a build that explicitly says it supports 1.21.7. If the second launch fails, you have isolated the change.
Next steps
Use the homepage for the complete feature, release, FAQ, and download overview. For a clean installation workflow, read the installation and troubleshooting guide.