Why do the correct paths to stylus files break (gulp-sourcemaps + browsersync)?

Question

gulp.js

There is a common style.style file. It has a number of imports from other .styl files. I remove any top imports and all paths of bottom imports break after browsersync reloads the page. If after that you manually update the page, then the paths are already normal.
Example:

Remove import normalize.styl (@import "assets/_normalize"). The import of this file is no longer there, but the paths point to it.

My gupfile:

var gulp         = require('gulp'),
    browserSync  = require('browser-sync').create(),
    uglify       = require('gulp-uglifyjs'),
    cssnano      = require('gulp-cssnano'),
    gulpIf       = require('gulp-if'),
    useref       = require('gulp-useref'),
    plumber      = require('gulp-plumber'),
    flatten      = require('gulp-flatten'),
    autoprefixer = require('gulp-autoprefixer'),
    ftp          = require('vinyl-ftp'),
    imagemin     = require('gulp-imagemin'),
    cache        = require('gulp-cache'),
    pngquant     = require('imagemin-pngquant'),
    spritesmith  = require('gulp.spritesmith'),
    stylus       = require('gulp-stylus'),
    sourcemaps   = require('gulp-sourcemaps'),
    pug          = require('gulp-pug');

gulp.task('pug', function(){
    return gulp.src(['app/pug/**/*.pug','!app/pug/**/_*.pug'])
        .pipe(pug({
            pretty: true
        }))
        .pipe(gulp.dest('app'))
});

// stylus

gulp.task('stylus', function(){
    return gulp.src(['app/stylus/**/*.styl', '!app/stylus/**/_*.styl'], {'base': 'app'})
        .pipe(sourcemaps.init())
        .pipe(stylus())
        .pipe(sourcemaps.write('.'))
        .pipe(flatten())
        .pipe(gulp.dest('app/css'))
        // .pipe(browserSync.stream())
});

// sync

gulp.task('browser-sync', function(){
    browserSync.init({
        server: {
            baseDir: 'app'
        },
        injectChanges: true,
        logLevel: 'debug'
    });
});

// watch

gulp.task('watch', function(){
    gulp.watch('app/pug/**/*.pug', gulp.series('pug'));
    gulp.watch(['app/stylus/**/*.styl','app/stylus/**/_*.styl'], gulp.series('stylus'));
    
    gulp.watch("app/css/style.css").on('change', browserSync.reload);
    gulp.watch("app/*.html").on('change', browserSync.reload);
    gulp.watch('app/js/**/*.js').on('change', browserSync.reload);
});

gulp.task('default', gulp.series(
    gulp.parallel('pug', 'stylus'),
    gulp.parallel('watch','browser-sync')
));