I had a similar problem with the lashing eyes on my truck. I wrote a short article on 'the other' demountable site which I'll replicate here when we are sure of the format.
But to paraphrase, the bucks on pickups are not deigned to accommodate the unintentional vertical forces they are subjected to caused by the lever action of the high demountable and can cause stress fractures. I'm sure the external 'button' fixing the Niche Marketing fit has similar issues.
As yours Andy, the 60mm plate wasn't man enough and was starting to pull up through the bed steel.
(BTW - I thought we discussed not mentioning 'porpoising' on these forums... there are 'special' places for that!

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I had the buck (side) mounting brackets reinforced (the fitting just in front of the rear wheel) to ensure the buck could take upward vertical forces.
The ✖ bracing bar (under the load floor crossing laterally about 150mm back from the bulkhead) re-welded as a couple of the plug welds showed cracking.
And finally had a super big 250mm x 120mm plate fitted on the rear eyes to act as a spreader with a strengthening longitudinal fold. (Front lashing eyes still using the small 60 x 60 square 'washers'- seems OK so far but will also get the super spreader treatment if any deformation occurs.)
So far... no more problems, but then again I haven't been on Greek roads since!
This is the cause of the problem (physics) the upward vertical force is the culprit
M12 Stainless eye bolts - originally both top and bottom spreader plates were this size, but that wasn't enough, so the underside has been beefed up.
Refabricated Buck Mount to deal with some stress fractures appearing here.
The super sized spreader plate on rear eyes, originals were 60mm x 60mm - these are 200mm x 100mm with a fold to give even more strength
This is what caused my problems
Richc - did you say that you are tying down to the original truck factory fitted lashing eyes? that's a huge no-no and may very well end in tears. Apologies if i have mis-read that in my usual hurry.
* Some of the photos here are being resized to fit the forum picture format, look later if still too big to see the important bits *