From the Log:
[05:19:19] [Worker-ResourceReload-0/WARN]: [fuzs.diagonalwindows.client.model.MultipartAppender#lambda$onPrepareModelBaking$2] Block 'Block{jello:deep_blush_stained_glass_pane}' is not using multipart models, diagonal connections will not be visible!

[05:19:19] [Worker-ResourceReload-0/WARN]: [fuzs.diagonalwindows.client.model.MultipartAppender#lambda$onPrepareModelBaking$2] Block 'Block{jello:mango_tango_stained_glass_pane}' is not using multipart models, diagonal connections will not be visible!

...repeats for all jello:color_stained_glass_pane

[05:19:19] [Worker-ResourceReload-0/INFO]: [fuzs.diagonalwindows.client.model.MultipartAppender#onPrepareModelBaking] Constructing diagonal block models took 143 milliseconds

Resulting in invisible textures:

My quick fix would be making multipart models for all the different stained glass pane colors to ship as an embedded resource pack, I just didn't know if there was any that could be done code wise without bloating load times. Regardless thank you for making this mod!

Such most likely comes down to the unique loading method imployed by Gelatin as the system attempts to duplicate the instances of loading the same model meaning the model is a DelegatingUnbakedModel and such will not be found thru the Diagonal Windows method of getting models.

I have attempted to add compatibility to properly modify the underlining model used within this commit b29c924

I got it working in dev but when in production it just crashes out right. Will need more time to find a fix...

Ok all fixed, had bad mixin logic and also wrong mixin setup values but should work.