I remember tripping over this gotcha when I was working on SkyControl back in 2017. The default format for FilterPostProcessor changed between JME 3.0 and 3.1, breaking my off-screen renders:
Porting tips - jME 3.0 to jME 3.1
Used R11G11B10F instead of RGB10_A2 for framebuffer format in Filters… · jMonkeyEngine/jmonkeyengine@200c875 · GitHub
RGB111110F uses the same amount of memory (4 bytes per pixel) as RGBA8. It’s not supported by all platforms. Its only advantage is greater color precision. I’d support changing the default format in FilterPostProcessor back to RGBA8.