在Angular中，自定义FormControl在FormGroup.reset()之后不更新视图的问题可以通过以下解决方法来解决：

1. 在自定义FormControl中实现ControlValueAccessor接口。

import { Component, forwardRef } from '@angular/core';
import { NG_VALUE_ACCESSOR, ControlValueAccessor } from '@angular/forms';

@Component({
  selector: 'app-custom-control',
  template: `
    
  `,
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      useExisting: forwardRef(() => CustomControlComponent),
      multi: true
    }
  ]
})
export class CustomControlComponent implements ControlValueAccessor {
  value: any;

  onChange: any = () => {};

  writeValue(value: any): void {
    this.value = value;
  }

  registerOnChange(fn: any): void {
    this.onChange = fn;
  }

  registerOnTouched(fn: any): void {}

  setDisabledState?(isDisabled: boolean): void {}
}

2. 在自定义FormControl组件中，添加一个reset方法，并在reset方法中更新视图。

import { Component, forwardRef } from '@angular/core';
import { NG_VALUE_ACCESSOR, ControlValueAccessor } from '@angular/forms';

@Component({
  selector: 'app-custom-control',
  template: `
    
  `,
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      useExisting: forwardRef(() => CustomControlComponent),
      multi: true
    }
  ]
})
export class CustomControlComponent implements ControlValueAccessor {
  value: any;

  onChange: any = () => {};

  writeValue(value: any): void {
    this.value = value;
  }

  registerOnChange(fn: any): void {
    this.onChange = fn;
  }

  registerOnTouched(fn: any): void {}

  setDisabledState?(isDisabled: boolean): void {}

  reset(): void {
    this.writeValue(null);
  }
}

3. 在使用自定义FormControl的组件中，调用FormGroup的reset()方法后，再调用自定义FormControl的reset()方法来更新视图。

import { Component, ViewChild } from '@angular/core';
import { FormGroup, FormControl } from '@angular/forms';
import { CustomControlComponent } from './custom-control.component';

@Component({
  selector: 'app-root',
  template: `
    

      
      Reset
    
  `
})
export class AppComponent {
  form: FormGroup;

  @ViewChild(CustomControlComponent) customControl: CustomControlComponent;

  constructor() {
    this.form = new FormGroup({
      customControl: new FormControl()
    });
  }

  resetForm(): void {
    this.form.reset();
    this.customControl.reset();
  }
}

通过以上步骤，自定义FormControl在FormGroup.reset()之后将会更新视图。当调用FormGroup的reset()方法时，同时调用自定义FormControl的reset()方法来更新自定义FormControl的值，并触发更新视图的操作。