I don't know how it works with the mgs5, but the MG4 is not consistent in this either. After the software update in January 2025, OPD suddenly worked at 100%. Then, after the summer, all of a sudden it refused to do so. It again went back to 97%-98% before I could activate it. But sometimes, it also worked at 99%.
I do charge more often at 100% than just the once-a-month balancing charge. It prevents me from paying expensive charge prices along motorways. I feel like there many variables causing you to use OPD at 100% or not. Like battery health status (> software changes charge behaviour) and the fact that 100% is a rounded number that may leave some absolute charge capacity on the table (99,5% when rounded off still leaves 0,374 Wh to top off with a 74.4 kWh net battery capacity). Another one is button mispress. If I select OPD when driving in town, bumpy rides have lead me to think I selected it. But the audio confirmation only told me .. I missed it. Depending when you find out it wasn't activated, charge levels may have dropped and you could think it is a different charge level when it actually activates. I found this out when I started asking my wife to change to OPD as she was sitting in the passenger seat.
This week I drove the ZS EV (loan car). Interestingly, I defaulted to level 3 and it never complained at 100%. But the regen reduced quite strongly, only to return to maximum after 10% or so. This could also happen to OPD and my MG4: the dash shows 100% and OPD is on, but the regenerated charge is lower....
Btw, also the MGS5 seems to roll backwards when stopping it on a steep slope with OPD on.