If you hit the spell button on the main game screen, and then right-click an available spell to pull up the spell info, you can hover over the bottom part of the screen and tooltips will pop up for memorized Cleric spells- I've provided a screenshot of the tooltip for Draw Upon Holy Might popping up. If you left click in these areas, you can hear a sound effect and checking the Priest Scroll confirms it was un-memorized. If you right click, it pulls up their information.
This occurs for Priest spells, Wizard spells, and innate abilities, and for all three it erases from the Priest Scroll of a character in the party. This bug is also present in BG:EE.