Upon inspection, it became apparent that the wrong trowel had been used to apply the adhesive. The adhesive manufacturer's instructions called for a 1/16" notched trowel to be used. There were few, if any, 1/16" trowel lines evident on the sub-floor beneath the loose flooring pieces.
Quite often, the right trowel size is used at the start of the installation. However, new trowels quickly wear down with use as they are scraped against the concrete sub-floor. If the trowel is not regularly replaced or re-notched as necessary, then the manufacturer's installation instructions will not be met and the product warranty will not be in place.