You probably need to manually point VS at the pdb file. It doesn’t get copied into the temporary layout used for running under the debugger (we skip it because tends to be a large file), so it’s not automatically located adjacent to the exe. This is a one-time thing - after you’ve done it VS will remember the association.
Open up debug->windows->modules, locate your exe (probably at the top), right-click and hit load symbols, then locate your pdb (in [Game]\Binaries\UWP64).