DISCLAIMER: we have a custom bundle and theme that are built from thin packages and include all products, which increases the risk of such style conflicts and other side effects.
A rendering regression was introduced in Bryntum 7.1.2 that causes Calendar columns in Gantt to display corrupted/broken layouts. The issue appears to be related to a CSS change that sets position: initial on .b-calendar-cell elements.
It is likely a result of this bug fix:
https://github.com/bryntum/support/issues/12487
Before:
After: