Watching changes in external folder

I'm trying to tell my application (frontend made on react) to rebuild if changes occure in backend (headless cms strapi), i figured out what exact strapi folder is modified each time I'm making any changes there, and trying to configure my pm2 script to watch it for changes.

This is how my json looks like

{
	"apps" : [{
	  "name":"frontend",
	  "cwd":"./shop",
	  "script":"npm",
	  "args":"start",
	  "watch":"../cms/.tmp/data.db",
	}],
}

But it ain't working out.

my folder structure looks like this:
root

| - pm2.json
| - shop
| - cms
       | - .tmp
               | - data.db

I tried to figure out how to make it via ecosystem, but also failed, had this error in error.log

(sharp:2783887): GLib-CRITICAL **: 21:48:43.712: g_hash_table_lookup: assertion 'hash_table != NULL' failed

my system information is next:
Strapi Version: 3.1.0-alpha.5
Operating System: Ubuntu 20.04
Node Version: 10.23.0
NPM Version: 6.14.8
PM2 Version: 4.5.1

Start with the pm2.json configuration and the watch setting shown in the issue, then check PM2's ecosystem configuration behavior for paths outside the app cwd. Reproduce the setup with shop and cms/.tmp/data.db; done means the frontend process rebuilds when that external file changes without the reported error.