Another thing to rule out (for those whose problem may resemble this but not be the same).
If drawing with vector lines make sure those lines are not zero thickness.
In Preferences > Visualization there is also an option to 'Show lines with zero thickness.'
That likely won't be the problem but it's good to rule that one out when troubleshooting.
Especially as that option is on by default (I think).
So to recap:
If using vector lines...
If lines are zero thickness..
If 'Show lines with zero thickness' is not enabled in Preferences...
Drawn lines may not appear on the canvas but will appear elsewhere (such as in preview/icon images).
As always, if experiencing problem with the official release it may be well worth the time to install a nightly release or the OTX or Morevna releases because they may contain fixes not included in the official release.