Classic whiteboard gets deleted if I share a different screen then come back to it

I'm on Zoom Version 6.3.11 (50104) on MacBook Air MacOS 15.3.2 (24D81)

I have been teaching online since lockdown and I use Classic whiteboard for notes.  I often switch to share a webpage or a pdf then come back to the whiteboard and continue the notes.  This works fine.  I share the notes with the student after the lesson.

Today, following the update, every time I switch back to Classic whiteboard all the notes are gone and I've got a blank whiteboard.  This is exceedingly disruptive as we normally carry on with the same calculations through the lesson and now all the work we've done is gone every time we come back after looking at a different screen, for example, the pdf of the problem we're working on.

I explain to the students that every once in a while Zoom releases an update that ruins everything, and it's happened again.

Please can someone direct me to the setting that makes the Classic whiteboard remain intact if you stop sharing it, share a different screen, then come back to it?

Thanks!