Transmission Temps 5VZFE Auto

Hi all, thought I would post this here rather than the general FB page as I imagine I would receive too many responses, plus I'm putting in some detail. Last week I was in the Kosciuszko NP and we were driving up a fairly long track for half hour or so. I wasn't in low range as wasn't really necessary and I have temp sensors on the transmission and engine. It's the type where you attach a sensor to the rocker cover bolts for the engine and the transmission pan outer bolts. The readings have always been consistent on normal driving conditions - between 80-90 for the engine and 75-90 for the transmission. The engine temps climbed up-to 100 and that is consistent with previous experience so all good there (I believe). The transmission climbed up-to 126 at which point the A/T oil transmission warning light came on. I stopped straight away as this has never happened before and let the temps drop. Once I started again, I noticed that the A/T temp would climb as we took any decent elevation at "slow speed" - and this is the key. Any up-hill climb at a reasonable speed kept the A/T temps at around 85-105 which makes sense as there is better air flow through the radiator (which was replaced about 1 year ago so new and coolant looks good and levels are good). What I noticed is if the A/T temp rises to about 105 and then I stopped with the engine running, the A/T temp kept climbing, however the engine temp was always stable. I wasn't using the O/D or ECT mode at any point. The drive home was fine as well, the A/T temp settled back to it normal range of 75-90.
The A/T was serviced 10,000 km ago - oil, gaskets replaced and not had any issues with its operation. Car has 280,000 km on it. Oil level in A/T is within spec on dip stick and look clean.
Really not sure what too make of it - is it a characteristic of the A/T to behave this way or should I get the A/T looked at in more detail?

Appreciate any feedback or comments.

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