Mac OS X 10.9.5
So far I've observed this on both my Lucienne playthrough and my Cammile playthrough. After I equip Willpower Amulets and begin an encounter, I find that my party members have more SP than they should.
I've only observed this when Ylianne is in the party with her party-wide +1 SP passive. For instance, Ylianne and Lucienne both have one Amulet equipped, Lucienne starts with 9 SP, and Ylianne starts with 6.
Can provide screenshots if needed.
Edit: Now I've also seen this error without Ylianne in the party. I equipped Lucienne with two Willpower Amulets, expecting her to enter combat with 5 SP. Instead, she started with 8.
Edit: And again with only one Amulet.