Autocomplete not showing inside buffer in fullscreen app

I am creating a fullscreen app, and one of the views acts like a form. Each of the inputs is created with a buffer.

One of the buffers (the "Category" input in the figure) has an autocomplete with words I take from a database. I know the autocomplete is working because if I start typing "Per" and press the arrow down it autocompletes to "Personal", which is an existing word. However, the word possibilities are not shown anywhere.

I tried increasing the height of the window for the buffer, but I got the same result.

I also tried replacing the Buffer with other widgets, such as a TextArea, but still no luck.

Am I doing something wrong or does the autocompletion only works at prompts?

The whole code for this app is somewhat large, but the portion for this buffer is smaller and provided here.

Start with the linked add_task_view.py around line 77 and compare the Buffer setup with the prompt-toolkit autocomplete behavior described in the issue. Reproduce the fullscreen form using the provided code, then determine whether suggestions can be displayed for a Buffer rather than only completed via the arrow key; done means visible autocomplete options appear while typing.