I've has a few times I had transmission limp mode from non transmission problem. I definitely would start with assuming it's not the actual transmission.

First time was wonky voltage from the alternator because the moron that installed it only tightened the B+ finger tight. (that moron was me)

The order times was from water getting into the main transmission connector on the back of the transmission.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)