Markers not loading correctly, console errors ("unsupported URL")

**I'm submitting a ...** (check one with "x")
- [ ] question
- [x] any problem or bug report
- [ ] feature request

**If you choose 'problem or bug report', please select OS:** (check one with "x")
- [ ] **Android**
- [x] **iOS**
- [ ] **Browser**

**cordova information:** (run `$> cordova plugin list`)

```
cordova-plugin-device 2.0.2 "Device"
cordova-plugin-fcm-with-dependecy-updated 4.6.4 "Cordova FCM Push Plugin"
cordova-plugin-googlemaps 2.6.2 "cordova-plugin-googlemaps"
cordova-plugin-inappbrowser 3.2.0 "InAppBrowser"
cordova-plugin-ionic-keyboard 2.2.0 "cordova-plugin-ionic-keyboard"
cordova-plugin-ionic-webview 4.2.1 "cordova-plugin-ionic-webview"
cordova-plugin-network-information 2.0.2 "Network Information"
cordova-plugin-splashscreen 5.0.2 "Splashscreen"
cordova-plugin-statusbar 2.4.2 "StatusBar"
cordova-plugin-whitelist 1.3.3 "Whitelist"
```

**If you use `@ionic-native/google-maps`, please tell the package.json (only `@ionic-native/core` and `@ionic-native/google-maps` are fine mostly)**

@ionic-native/google-maps v4.21.0
@ionic-native/core v4.20.0

**Current behavior:**
After map is loaded sometimes the markers are shown but as soon I move the map (zoom or drag) the markers disappears and I get these errors in console ("unsupported URL").
```
2020-05-19 10:22:59.139571+0200 AppAgenti[833:22663] Task <270E75CF-877A-4C2B-BBC0-5DCC8B8F7158>.<1> finished with error [-1002] Error Domain=NSURLErrorDomain Code=-1002 "unsupported URL" UserInfo={NSUnderlyingError=0x600000bd16e0 {Error Domain=kCFErrorDomainCFNetwork Code=-1002 "(null)"}, NSErrorFailingURLStringKey=ionic://assets/imgs/markers/map/allevamento-bufale.png, NSErrorFailingURLKey=ionic://assets/imgs/markers/map/allevamento-bufale.png, NSLocalizedDescription=unsupported URL}
2020-05-19 10:22:59.141681+0200 AppAgenti[833:11881] [fail] url = ionic://assets/imgs/markers/map/allevamento-bufale.png
2020-05-19 10:22:59.143074+0200 AppAgenti[833:11881] (error) Can not load image from 'ionic://assets/imgs/markers/map/allevamento-bufale.png'.
```
Other times the markers are not loaded correctly from the beginning and I get the same errors in console.

**Expected behavior:**
Markers correctly loaded

Tested on iOS 13.4.1. How can I fix this issue? Thanks