I first encountered this issue in Kokua Viewer 7.2.3 but it's also occurring in the latest official viewer so I suspect this is something that came in from Linden Lab side.
Steps to reproduce:
  1. Log into Second Life as main account, customise toolbars.
  2. Log out, log in again as alt account.
  3. Log out, log in again as main account.
  4. Toolbars have been reset to defaults.
Thankfully, it's only the toolbars resetting, other settings and presets are unchanged.