Why do dash to dock icons disappear after sleep mode? Ubuntu 21.10

The reason is that your extension is not compatible with the version of Gnome Shell that comes with your upgraded Ubuntu version.

If you install software outside of the official repositories, then you are responsible for whether it works or not. Dash to Dock is not supported software in Ubuntu. Thus, when you upgrade to a newer Ubuntu version, it may break.

Remove the Dash to Dock extension. If there is an updated version, you may take the responsability to install that one instead. However, it is safer to just use the Ubuntu Dock extension. That is based on Dash to Dock and packaged specifically to work with your current Ubuntu version.

I was also facing same issue, but once I disable built-in "Ubuntu-Dock", in the extensions, everything seems to be fixed.