simply logging VFR instructional time as IFR
If a flight was VFR then no IFR time can be logged.
I've heard that some FI's are logging also student's IFR time during PPL/CPL
If a student had IFR time then, as they were on the same flight, the instructor must also have had IFR time !
PPL and CPL are VFR Licences and require no IFR instruction.
(Any IF elements easily being conducted in VMC)
The problem is in the wording of Part-FCL that only requires Instructional Privileges if the flight is for a Licensing action (Licence/Rating Issue/Renewal/Revalidation) - not for teaching a subject.
I have heard about that too, the instructor told me he wrote the instruction time in the aeroplane log book as PIC and in his own log book as PIC and IFR, not as an instructor.
An Instructor
is PIC (otherwise they are just a passenger).
If a PPL or CPL student is on board then, I would suggest, no IFR flight can be logged as the Instructional Privilege being used on that flight does not entitle IFR Instruction.
It may be that some Licence instructional flights do have a small amount of IFR flight time in them (using the PIC's qualification) eg to get through some bad weather or above a cloud layer. But, as this does not form part of the instruction that the student is receiving (and paying for) I would expect it to kept quiet, and not be logged as such.
Any Logging must be the same in both the Instructor's and Student's Log Books, and must match the Student's Training Record.