The following paragraph is quoted from the chapter on changed behaviour within the Oracle APEX 26.1 Release notes
The existing Static ID property for Regions, Buttons, Interactive Grid, and Interactive Report Columns is renamed to HTML DOM ID to better reflect its purpose in the DOM and to avoid overlapping with the newly-introduced Static ID used in APEXlang application exports.

Acknowledgements
Thanks to this post on LinkedIn from John Scott which made me aware of this change.