请具体描述问题出现的流程，并提供能复现问题的简单代码片段（https://developers.weixin.qq.com/miniprogram/dev/devtools/minicode.html）。

由于开发者工具有 BUG，只好手工粘贴代码在这里

注：经测试，貌似些 BUG 只会在开发者工具出现，手机端好像不会出现

js 文件

// js
Page({
  data: {
    canvas_info: {
      width: 2382,
      height: 2406,
      cols: 6,
      rows: 3,
      uWidth: 397,
      uHeight: 802
    },
    img: {
      url: "https://chiz.img.jwwvip.com/images/6252dbea397e68b740d1e21f.jpg?x-oss-process=image/resize,m_mfit,l_800,s_395",
      height: 800,
      width: 395,
    }
  },
  onLoad() {
    const that = this;
    wx.createSelectorQuery().select('#scheme')
      .fields({
        node: true
      })
      .exec(res => {
        const canvas = res[0].node;
        const canvas_info = that.data.canvas_info;
        const img = that.data.img;
        that.draw_image(canvas, canvas_info, img.width, img.height);
      });
  },


  draw_image(canvas, canvas_info, sWidth, sHeight) {
    const that = this;
    const ctx = canvas.getContext('2d');
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    canvas.width = canvas_info.width;
    canvas.height = canvas_info.height;
    const img_opts = that.get_img_opts(canvas_info, sWidth, sHeight);
    let success_cnt = 0;
    wx.showLoading({
      title: '正在生成中',
      mask: true,
    });
    img_opts.forEach(opt => {
      const img = canvas.createImage();
      img.onload = () => {
        ctx.drawImage(
          img, opt.sx, opt.sy, opt.sWidth, opt.sHeight,
          opt.dx, opt.dy, opt.dWidth, opt.dHeight,
        );
        success_cnt += 1;
        if (success_cnt === img_opts.length) {
          wx.canvasToTempFilePath({
            canvas: canvas,
            quality: 0.8,
            fileType: "jpg",
            success: res => {
              console.log(res.tempFilePath);
              that.setData({
                path: res.tempFilePath
              });
              wx.hideLoading();
            }
          });
        }
      };
      img.src = opt.url;
    });
  },


  get_img_opts: function (canvas_info, sWidth, sHeight) {
    const that = this;
    const result = [];
    for (let i = 0; i < canvas_info.rows; i++) {
      for (let j = 0; j < canvas_info.cols; j++) {
        result.push({
          url: that.data.img.url,
          sx: 0,
          sy: 0,
          sWidth: sWidth,
          sHeight: sHeight,
          dx: canvas_info.uWidth * j,
          dy: canvas_info.uHeight * i,
          dWidth: sWidth,
          dHeight: sHeight,
        });
      }
    }
    return result;
  },


  preview () {
    const that = this;
    wx.previewImage({
      urls: [that.data.path],
    });
  },


  save() {
    const that = this;
    wx.saveImageToPhotosAlbum({
      filePath: that.data.path,
    });
  }
});

wxml 文件

<view class="img_box" style="height:432rpx;">
    <canvas bindtap="preview" id="scheme" type="2d" style="width:400rpx; height:400rpx"></canvas>
</view>


<button type="primary" bindtap="save">保存</button>

wxss 文件

.img_box {
  width: 100%;
  display: flex;
  justify-content: center;
  align-items: center;
  border-bottom: 1px solid rgba(128, 128, 128, 0.2);
}