I'm a bit surprised that Apple doesn't have tools to diagnose memory leaks that can be made available to customers. We call this Serviceability Engineering and I used to have that job. The job entailed building tools that made it easier to diagnose problems in the field. One example of this would be to write a crash dump analyzer that can diagnose problems based on the stack trace or that would provide a summary form of things that engineers want to quickly look at.
There are commercial tools to sniff for memory leaks but they're usually licensed for development organizations. It would be nice if Apple could engineer tools into macOS to do allocation and free and then analyze the calls to see where memory is leaking by the customer turning on a debug model which generates trace output that could be sent back to Apple for analysis.
That was also my former job at another company. It works better for an outright immediate crash caused by (for example) an invalid pointer reference than a gradual problem like a memory leak. There are ways an iOS or MacOS developer can have *crash* info automatically sent, provided the customer opts in.
By contrast a memory or resource leak leak is more gradual. Anyone can take a spindump of a hung or deranged process using Activity Monitor, but interpreting that generally requires source code access and a deep internal understanding of the application. Such people are hard to come by and typically would be working in new product development, not support. Companies often view the support org. as a cost center whereas product development is a profit center. It is rare to have even a tier-3 support person with source code access, deep internals knowledge and ability to debug customer problems at the source level.
For app developers with source code access, Apple has various ways to debug a memory leak. That is covered in various WWDC presentations. But the emphasis is on a developer locally debugging their own application, not remotely troubleshooting a deployed app. The common approach is problems in the field require the customer to develop a packaged procedural problem replication, send that to Apple, then they will examine it. But there is a class of problems that are not crashes, cannot be replicated, but still require support.
Xcode can use a facility called "instruments" which a developer with source code access can embed in the app to track or log parameters. In theory they can build a stand-alone instrument package which would be sent to a customer to gather debug info, but I've never heard of any FCP customer who had that done.
In the OP case, it was FCP which often uses 3rd-party plugins, often multiple plugins. Prior to the FxPlug 4 framework, those ran "in process" or within the FCP address space. Any bug in any plugin could destabilize or crash FCP. The new FxPlug 4 framework allows plugin developers to modify their code to run "out of process" and communicate with FCP via an XPC. In theory that should make the plugin ecosystem more stable. However under FxPlug 4 the developer assumes additional responsibilities about multi-thread synchronization.
I have never seen any white paper or WWDC talk about the procedure for debugging a stability problem in a deployed host app and multiple plugins, where no one person has access to all the source code (or even debug symbols). Considering how pervasive application-specific plugins are, you'd think there would be more info on that.
Obviously the ideal approach (if achievable) is for the customer to find a reproducible procedure and small, portable data set which causes the problem. But if this requires 3rd-party multiple plugins it can devolve into a finger-pointing situation. For a customer who encounters a serious non-crash problem which involves multiple 3rd-party plugins and is both intermittent and impossible to reproduce, they can try to replicate it without the plugins. If that's not possible it's a difficult situation.