This looks like the same issue that I had (4.25.2).
The Synthesis plugin happens to load too late in the engine loading process on startup, which makes it unavailable when loaded after initially being used. Changing the .uplugin file on both plugins to have the "LoadingPhase": "PreDefault" makes these plugins load in before your project files, fixing the issue. .uplugin files are interpreted at load-time, so you don’t need to recompile the engine. The .uplugin file on this plugin is in its folder in (Engine Dir)/Engine/Plugins/Runtime.
Keep in mind that because this change is in engine files, it might not be captured in your source control software. What I plan on doing is making a script that automatically changes out the file for all of my devs’ engine download.