Even though the mod already provides Dynamic Trees support for Flowering Oak trees, said trees are not replaced in the Sacred Springs biome.

Can confirm..... any hope of a fix?

Not just the Flowering Oak, but also the Sacred Oak in Sacred Springs biomes are vanilla trees rather than dynamic.

BiomesOPlenty-1.12.2-7.0.1.2441-universal
DynamicTrees-1.12.2-0.9.5
DynamicTreesBOP-1.12.2-1.4.1e
forge-1.12.2-14.23.5.2838

Have you tried setting large Oaks enabled to false in the config file for the Sacred Springs biome? Noticed that that was the only way to seemingly get the base BOP flowering oaks to stop spawning in that biome.

It should be in the '.minecraft\config\biomesoplenty\biomes\defaults\biomesoplenty' folder. Though you will have to place a copy of it with the changes into the folder that has a blank file telling you to put the config files there.

Also I think the Sacred Oaks were one of the trees that weren't going to get a dynamic version of due to the large amount of leaves/wood shoved into the upper section likely making a dynamic version hard to make look right.