Thermal Core Tip Stalling

There have been several incidents and accidents over
the last few years when gliders, especially big span
ones, have suddenly spun or spiral dived when entering
or manoevring in strong narrow thermals. I am therefore
wondering if pushing a wing down into a strong thermal
core could cause a momentary tip stall? Wing going
down plus air coming up could cause the stalling angle
of attack to be exceeded.

Derek Copeland