Update
The issue is Victor Engine's Basic Module and Battle Motions, specifically these 2 functions found in Basic Module
Spriteset_Battle.prototype.sortBattleSprites
Spriteset_Battle.prototype.compareBattleSprites
These 2 functions are called during the update function for Spriteset_Battle which sorts all sprites based on z,h, and y values. Foreground battlebacks added by your plugin are sorted to the end of the list. Presumably there is a small timing conflict which is why for an occasional frame or 2 the sprites were not sorted yet and the foreground displayed as anticipated. For now I have commented out Victor's Spriteset_Battle.prototype.sortBattleSprites function. I do not know what unintended effects this will have for fellow Victor Engine users, but it does appear to solve this compatibility issue with your plugin. I don't think there's any updates to be made to your plugin, just for anyone using VictorEngine, try commenting out that functions definition. I will update again if I find it breaks something else.