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I had to copy that kind of bodge on a floor recently and was surprised how well it worked. The flooring on top was 18mm not 6mm though so that might play a role.
It is definitely a bodge and I was surprised that anyone would have done it that way but it worked and the client wanted it reinstated after a flood so I did recreate it.
It's all to save lifting the boards and dealing with whatever else you find under there really.
Agreed on self levelling. The strip approach had occurred to me but suspected it was on, if not all the way along, the bodge scale.
I've got plenty of 6mm offcuts around so might try and dry fit something and see how it sits. Thanks!