From the description and screenshots I suspect this is the problem: There's a fundamental issue with projected screen-space decals where texture coordinate derivatives have high values at geometry borders due to the depth discontinuity, which ends up sampling low resolution mips at geometry borders causing the artifact. There's no proper fix for this but I "fixed" (or rather reduced) the issue in HDRP on emissive decals some time ago, with manual LOD calculation and clamping the LOD with depth derivatives (DecalData.hlsl, line ~35). Here's some details about the fundamental issue: https://bartwronski.com/2015/03/12/fixing-screen-space-deferred-decals/