Is there any official CODESYS documentation on automatic type conversion?

If CODESYS relies on the IEC 61131-3 standard for the above, are there any online resources containing or citing that part of the standard?

The closest I have found is a reference to the issue and the standard here.

Automatic Type Promotion
Elementary Types have a priority defined by the position in the above table. For example type REAL is higher priority than type INT.

When two different types are used in an operation automatic type promotion occurs. The inputs to the operation are all promoted to the same type as the highest priority input. For example if an INT is added to a REAL, the INT value is promoted to a REAL before the addition operation. The return value from this operation will be REAL.

Standards Compliance
IEC 61131-3 Second Edition: Table 10.
IEC 61131-3 Third Edition: Table 10.