Just confirmed, Doompriest cast Mass Possession, Sandlashed extended it, Athel cast Blessing and attacked and ended turn with no buffs.

Note:  Also, just noticed it happens on waves too, not just the boss.

Note 2:  When she casts Blessing, she drops ALL attack, even her own, and attacks with NO attack buff.

Note 3:  Occurs without Sandlash extend prior to turn as well.

Note 4:  Had 2 charges of Mass Possession, cast A1, had one charge Mass Possession left, cast A3 into A1 but A1 made with no buff and ended turn with no buff, not even her own buff she had just cast.

Note 5:  Appears to be Mass Possession issue, Valerie buffed party and though Blessing overwrote it Athel still had an attack buff after her turn.

Ok, so Possession is 50% and both Energizing Presence and Higher Blessing are 25% attack buffs.  I'd think that even if Athel overwrote or burned the duration of Possession by using an additional turn and/or casting a weaker version she'd still have the 25% version remaining for 1 turn after casting and attacking. 

Took Doompriest out so there was only 25% buffs and Athel both attacks and ends her turn with Blessing intact.  The issue is overwriting a 50% attack buff with a 25% attack buff.  Is she overwrites another 25% everything works fine.  If she tries to overwrite a 50% buff she loses the 50% and doesn't get a 25% for either the attack or the next turn.