Structs aren't being GCed despite WeakHashMap usage

### Environment Information

```markdown
n/a
```

### Bug Description

Some structs appear to be getting updated despite being in long-dead local variables, indicating a memory leak of some sort.

### Steps to Reproduce

Change the template of a struct that was in a local variable. See if the modifed message appears.

### Errors or Screenshots

_No response_

### Other

_No response_