You might be missing some debug symbols 
What if you run once coredumpctl -1 gdb plasmashell.
If you had a previous crash in plasmashell it will download the necessary symbols. It should offer to download and install debug symbols, accept all of them.
Once you get to the line (gdb) link, type in “exit” then enter, or Ctrl+D to quit.
Otherwise it should mean, your debug symbols for plasmashell or other packages is not up-to-date anymore, and you might need to do a new heaptrack capture.
Or there is another issue, but I couldn’t say what. Maybe your distro has custom plasma packages without debug info matching your binaries available, hard to say.