ionic 4 nested tabs ion-tab-button 'tab' property is not working properly for nested tabs

Question

I am using ion-tabs for my frontend angular ionic app. I am using nested tabs, So 1st ion-tabs have 3 tabs, Tab 1, Tab 2, feature-tabs and feature-tabs has 3 tabs, Tab 4, Tab 5, Tab 6. I have used the Angular routing.

So the routes look like the following:

1. tabs/tab1,
2. tabs/tab2,
3. tabs/feature-tabs/tab4,
4. tabs/feature-tabs/tab5,
5. tabs/feature-tabs/tab6

The issue when feature-tabs is selected Tab 4 is the active route by default but the property 'tab' in the ion-tab-button when kept similar to the path (i.e. tab ="tab4") in the routing the default as recommended Tab 4 works but selecting/click on the tabs gives an error of Cannot match any routes. URL Segment: 'tabs/tab1'. This means that the 'tab' property of the ion-tab-button is referring to the parent tabs. When I change the property 'tab' in the ion-tab-button to "feature-tabs/tab4" (not recommended), everything works but the --color-selected CSS property of the ion-tab-button doesn't work. But my question is why the tab property is referring to the patent tabs and how can I refer back to the child feature-tabs?

I have already tired to change the CSS property but it simply doesn't work,

ion-tab-button{
--color-selected: #a0a;
}

This is my routing for the tabs,

import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
import { TabsPage } from './tabs.page';


const routes: Routes = [
    {
        path: '',
        component: TabsPage,
        children: [
            {
                path: 'tab2',
                loadChildren: './../tab2/tab2.module#Tab2PageModule'
            },
            {
                path: 'tab1',
                children: [
                    {
                        path: '',
                        loadChildren: '../tab1/tab1.module#Tab1PageModule'
                    }
                        ]
                    }
                ]
            },

            {
                path: 'feature-tabs',
                loadChildren: '../feature-tabs/feature-tabs.module#FeatureTabsPageModule'
            },

        

            {
                path: '',
                redirectTo: './tabs/tab1',
                pathMatch: 'full'
            }

        ]
    },
    {
        path: '',
        redirectTo: '/tabs/tab1',
        pathMatch: 'full'
    }
];

@NgModule({
    imports: [RouterModule.forChild(routes)],
    exports: [RouterModule]
})
export class TabsRoutingModule { }

This is tabs html,

<ion-content>
  <ion-tabs>
    <ion-tab-bar slot="bottom">
      <ion-tab-button tab="tab1">
        <ion-label>Tab1</ion-label>
        <ion-icon name="pricetags"></ion-icon>
      </ion-tab-button>
      <ion-tab-button tab="tab2">
        <ion-label>Tab2</ion-label>
        <ion-icon name="podium"></ion-icon>
      </ion-tab-button>
      <ion-tab-button tab="feature-tabs">
        <ion-label>Feature tabs</ion-label>
        <ion-icon name="cash"></ion-icon>
      </ion-tab-button>
    </ion-tab-bar>
  </ion-tabs>
</ion-content>

this is my feature-tabs routes:

import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
import { FeatureTabsPage } from './feature-tabs.page';

const routes: Routes = [
    {
        path: '',
        component: FeatureTabsPage,
        children: [
            {
                path: 'tab4',
                children: [
                    {
                        path: '',
                        loadChildren: './../tab4/tab4.module#Tab4PageModule'
                    }
                ]
            },
            {
                path: 'tab5',
                children: [
                    {
                        path: '',
                        loadChildren: './../tab5/tab5.module#Tab5PageModule'
                    }
                ]
            },
            {
                path: 'tab6',
                children: [
                    {
                        path: '',
                        loadChildren: './../tab6/tab6.module#Tab6PageModule'
                    }
                ]
            },
            {
                path: '',
                redirectTo: '/tabs/feature-tabs/tab4',
                pathMatch: 'full'
            }
        ]
    },
    {
        path: '',
        redirectTo: '/tabs/feature-tabs/tab4',
        pathMatch: 'full'
    }
];

@NgModule({
    imports: [RouterModule.forChild(routes)],
    exports: [RouterModule]
})
export class FeatureTabsRoutingModule {}

this is the html for the feature-tabs:

<ion-content>
  <ion-tabs>
    <ion-tab-bar slot="top">
      <ion-tab-button tab="tab4">
        <ion-icon name="calendar"></ion-icon>
        <ion-label>tab4</ion-label>
      </ion-tab-button>

      <ion-tab-button tab="tab5">
        <ion-icon name="contacts"></ion-icon>
        <ion-label>tab5</ion-label>
      </ion-tab-button>

      <ion-tab-button tab="tab6">
        <ion-icon name="information-circle"></ion-icon>
        <ion-label>tab6</ion-label>
      </ion-tab-button>
    </ion-tab-bar>
  </ion-tabs>
</ion-content>