Our two XC70 models, one D5, one D4, does something similar. But at lower revs. You have to be somewhere down below 1300 r/min or so (depending on load), when they sometimes do a smaller, hydraulic downshift by just releasing the lock-up. Either completely or at least somewhat (limited slip).
This is to avoid vibrations at certain load levels. Maybe your older model (ours are 2012 and 2016, with the six speed gear box) does this at a higher engine speed?
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