THe spike is probably electrical noise in the system, possibly (but not likely) a poor solder connection somewhere in the voltmeter circuit part of the system (usually inside the controller, but possibly within the display; usually a voltage divider and sometimes an op-amp for buffering before going into the MCU).
How the display metering system works will determine what readings it gives and stays at. If it is poorly designed it could keep increaseing the voltage reading after a spike, instead of properly averaging.
But you have another problem, if that final charge voltage of 82.7v is correct:
The voltage on your battery after charging before riding indicates it isn't balanced. At 4.2v per cell, for a 20s battery, it should read 84.0v. An 84.5v charger works for that; your BMS shuts off charging once any cell reaches it's HVC (which is probably 4.2v).
But if the cells are not well-matched and the pack is not well-built (both very common problems), some cells (or groups of them) will have less capacity and higher resistance, so they will reach HVC first, and shut off charging before the rest of the cells are full, so the total pack voltage is low.
With a final charge voltage of only 82.7v, that's 84v - 82.7v = 1.3v. If that is distributed among all the other groups, it's not such a big deal, as little as 0.07v difference per cell from full. But if it is on just one group, that's pretty seriously imbalanced, as it would mean that the rest of the groups are at 4.2v while one is at 2.9v, which is very very low (empty!). Could be on any number of groups.
I would recommend opening the pack up, and checking the voltage on each group at the BMS connector itself (preferably on the solder joints on the back of the BMS, as that also verifies the cell connections to the BMS so you will know it is sensing them all "correctly").
THen list each of those group voltages here in this thread, starting at the most negative, and working your way up to the most positive, for all 20 groups. This will help us help yo find whcih group(s) cause the problem, and see if it is a significant problem, or one you can fix easily by leaving the battery on the charger longer (usually at least several hours--if it's a severe imbalance, it can take days or even weeks depending on the cause of the problem).
If it's a single group or only a few that are really low, you may have a cell defect problem, or worse you could have a BMS with stuck-on balancers that are killing your cells continuously whether you use the pack or not.
Or you may have a BMS with no balancers at all, and poorly-matched cells that are not equally capable; in this case the problem will get worse and worse over time, and faster the further it is discharged each time, under higher loads.