Hi! One of my users reported this bug to me, and I was able to replicate it:
Context:
Form+Chat document uploads work fine when submitting documents through the form (see images 1 and 2)
Image 1: Uploading file through form interface 
Image 2: Testing to see if file was correctly read
The document is read in full, and all contents are extracted.
The Bug
However, uploading the form and then trying to upload the same file causes it to bug, not being able to read the document correctly (see image 3). It seems to be able to read the first few lines but not much else.
Image 3: Form submitted without the document as input → Document uploaded during chat phase → Bug 
The same document was used in all three images. I’ll leave the file here in case it helps debugging: link to file.