See screenshots:
in 2.4.0 all OK:

in 2.5.0

At first I thought that the problem in forge MinecraftForge/MinecraftForge#4746

I'm not having any issues with the light levels... Is optifine present?

Yes, without optifine in 2.5.0 light looks like fine. With optifine, the flowers look faded :)

dont use optifine it breaks rendering in a lot of mods

Well, nothing I can do about optifine breaking the lighting. I forget if optifine has a config to re-enable the forge lighting pipeline, but if it doesn't, oh well.

How did it happen that in 2.4.0 everything is OK?

2.4.0 doesn't dynamically create the flower pot models at runtime, it literally was a couple thousand line json file hardcoded to a bunch of things. The vanilla lighting pipeline (which optifine appears to re-enable) has issues with dynamic generation, apparently. If OF has a config to enable the forge pipeline, it'll work just fine.

Thanks for answer!

There is a pretty old bug open for Optifine/Forge here if you want to follow it in case something happens:
MinecraftForge/MinecraftForge#3091

Force disabling AO appears to cause it to be fine under the Vanilla pipe, but I didn't ever test it with OF.
@KorDum You should go check with OF under 2.5.2 and see what it looks like.

No, no changes.

Huh. Wonder what OF does then. Disabling the Forge pipeline doesn't cause the lighting errors for me after the AO changes, but OF already disables the forge pipe... Oh well.

Is there any small hope for correction? :)

You could try disabling the forgeLightPipelineEnabled in Forge.cfg, but that shouldn't do anything unless optifine's modid isn't actually optifine.