I've also noticed this and thought it was me messing something up
Yep, heavy into sp2 and I seem to lose everything.. not consistent though, very intermittent
Unfortunately not a bug, but a really strange design choice in his kit. He only gives the opponent a combo shield while they're at 100 or less combo, so if you hit them while they're above that their combo gets reset and you lose all your ramp.
For example, if the opponent has 81 combo (as in you got hit once at max ramp) and you cancel a heavy punish into sp2, the opponent goes to 101 combo and loses the combo shield as soon as the special activates. When the first hit lands, the opponent loses their combo before it can be consumed. As a samurai stan, the fact that the opponent can lose their combo shield is probably my least favorite part of his kit