I can reproduce this in your example:
https://www.bryntum.com/examples/scheduler/workingtime/
Go the the console and call scheduler.zoomToFit()
The scheduler does not zoom to extents as it does when workingTime is not set.
It looks to me like it's attempting to zoomToFit as if all days are present instead of working days only.
Support Forum
Zooming is not supported when workingTime is enabled. Feature request here: https://app.assembla.com/spaces/bryntum/tickets/8791-add-zooming-support-when-workingtime-feature-is-enabled/details
Pavlo Miklashevych
Sr. Frontend Developer