Nether Portal texture is broken when using fancy nether portal option in Complementary Shaders. I tested but it only happens when both Lambdabettergrass and EBE are enabled along with Complementary Shaders v4.2 dev3 with fancy portal on Iris.

Mods:
fabric-api-0.37.2+1.17.jar
fabric-language-kotlin-1.6.3+kotlin.1.5.21.jar
iris-mc1.17-1.1.1
indium-1.0.0+mc1.17.1.jar
starlight-1.0.0-RC3+fabric.1.17.x.jar
lithium-fabric-mc1.17.1-0.7.3.jar
lambdabettergrass-1.2.0+1.17.jar
enhancedblockentities-0.4.1+1.17.jar

Complementary Shaders v4.2 dev3 on Iris ; Fancy Nether Portal Enabled through common.glsl.

So I reduced the mod list to:

Case 1:
fabric-api-0.37.2+1.17.jar
iris-mc1.17-1.1.1
indium-1.0.0+mc1.17.1.jar
lambdabettergrass-1.2.0+1.17.jar
enhancedblockentities-0.4.1+1.17.jar
Shaders: Complementary Shader 4.2 dev2-dev5(Fancy Nether Portal enabled through common.glsl)

Result:

Case 2:
fabric-api-0.37.2+1.17.jar
iris-mc1.17-1.1.1
indium-1.0.0+mc1.17.1.jar
enhancedblockentities-0.4.1+1.17.jar
Shaders: Complementary Shader 4.2 dev2-dev5(Fancy Nether Portal enabled through common.glsl)

Result:

Case 3:
fabric-api-0.37.2+1.17.jar
iris-mc1.17-1.1.1
indium-1.0.0+mc1.17.1.jar
lambdabettergrass-1.2.0+1.17.jar
Shaders: Complementary Shader 4.2 dev2-dev5(Fancy Nether Portal enabled through common.glsl)

Result:

Case 4:
fabric-api-0.37.2+1.17.jar
iris-mc1.17-1.1.1
indium-1.0.0+mc1.17.1.jar
Shaders: Complementary Shader 4.2 dev2-dev5(Fancy Nether Portal enabled through common.glsl)

Result:

EBE does not even remotely touch nether portals, and as far as I know neither does LambdaBetterGrass
Fabric language kotlin is also not required by any of the mods in your mod list as far as I know either, you don't need it installed
Please narrow down your mod list until you can figure out which mods SPECIFICALLY cause the issue
Also join the discord server to post updates when testing (link on mod page)

It got fixed with new development version of Iris and Complementary Shaders. Thanks for support.