Per the PHP 7.1 UPGRADING document:

Session ID is generated from CSPNG directly. As a result, Session ID length could be any length between 22 and 256. Note: Max size of session ID depends on save handler you are using.

Our database table does not accommodate this potential maximum length. MySQL has a length of 191 and the other databases are capped at 200.

Since apparently this is handler dependent, it may be possible with the PHP API to set a maximum length via our handlers if we cannot consistently lengthen our columns for the full range. Since we are always storing session data to the database, even when the database handler is NOT in use, it will have to be a global change.