Just happened to me again, first time ever while zoomed in. I was in SU-85B and locked while watching an enemy Easy 8 die. I've had the lock with:
Jimbo's on and off so not that.
Don't run Auto Aim mods so not that.
Don't run spot indicators so not that.
Battle Calc on and off so not that.
Multi hit log on and off so not that.
I'm thinking SPG battle assistant, Team HP pool (Locostan), Safe shot or destroyed vehicle skins.
Pared it down to these as they have all been enabled during this. I'm leaning towards destroyed vehicle skins or HP bar as lock occurs when a tank dies. Will disable both and see how it goes. Unfortunately the lock occurs infrequently so testing will be slow and laborious.