Form state values missing after file upload

Problem/Motivation

I have a webform handler that populates a dependent select using a view, so that after selecting a first value, a second select populates using a view with the first value as a parameter. The webform also has a file upload field.

After selecting a value for each dropdown, I upload a file. Then in the webform handler, the form state no longer has the values it had before, making the webform handler behave incorrectly.

Steps to reproduce

Having a dependent select is not necessary.

Create a webform with a select and a file field, and create a custom webform handler class with an alterForm function.

Inside the function, $var = $form_state->getValue('my_select'); should be used to get the value of the select. Set a breakpoint here.

If you go to the webform, select a value in the dropdown, then upload a file, the webform handler alterForm will run and will not show the value at the breakpoint. It will be NULL.

Despite the file upload using AJAX to only reload the file upload element, the form state is set to only contain the file-related values, and it doesn't contain any values that it contained prior to file upload.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes