You were not capped at 32A, that is for certain. Thirty-one kW = 31,000 watts; divided by the putative 32 amps, gives nearly 1,000 volts. Not possible at a Supercharger. They are limited to about 400 volts. The 32A is the onboard charger that converts the AC from the wall to DC for storage.
You are correct that at 45% SOC you should be pulling more than 31kW. First off, if you did not already know, classic Superchargers are paired, and two stalls share one charger. The first to plug in receives the maximum charge rate, and the second to charge receives sloppy seconds. As the first car fills and gets into the taper, the second car receives a higher rate of charge. So, if you plugged into a stall, (say 2B) and stall 2A was charging, then you would receive a much lower rate if the first car was pulling 85kW.
If you were not sharing a pair of Superchargers, there is really no way of knowing why your charge was so limited. Many of us will unplug and move to a different set of Superchargers to see if the pattern recurs.
There could be an issue with one set of stalls that restricts the power. If the temperature is cold, the battery might be too cold to receive the maximum rate until it heats up. Maybe there is some issue with the utility, and the utility has reduced the power available to the entire site.
The best thing is to try to contact Tesla and report your observation. The attendant might be aware of the issue already, or might make a note to have this location inspected by the Supercharger team.