Django filter qs contains filename not working with underscores

Question

my goal is to filter if the name of an uploaded document contains a search (a simple string from an input field submitted by the user). Strangely this works for long substring but not for short ones.

Example

My filter query is the following:

Tab.objects.filter(document__file__icontains=search)

where Tab is in simplest form a Model class like:

class Tab(models.Model):
    name = models.CharField(max_length=50)

and Document is another Model class wit a relation to Tabs and a file field.

class Document(models.Model):
    tab = models.ForeignKey(Tab, on_delete=models.CASCADE)
    file = models.FileField(upload_to=custom_location)

Weird behavior

I've uploaded a file called example_test_123.pdf and if my search = example_test or test_123.pdf it results in a hit like expected.

However, if search = test the filter doesn't find the file.

Is this behaviour known or am I missing something here? Is it possible that the built-in name file is doing some trouble?

Edit

1. I changed file to document_file without success
2. Even _test or test_ is working

Edit #2

@ Iain Shelvington (Comment)

This is no special function... only setting the subdirs the file should be saved

from django.utils.text import slugify

def custom_location(instance, filename):
    return os.path.join('documents', slugify(instance.tab.name), filename)

But your question is pointing me in the right direction for a workaround (thanks):

I'm doing an annotation before filtering. companies is in this case a ManyToManyField to another model:

tabs = Tab.objects.all().annotate(num_companies=Count('companies'))

and then filtering for some stuff like

tabs = tabs.filter(num_companies__gte=3)

Removing these lines results in a correct filtering. So I'm doing the annotations after filtering for the filename.

But I'm still curious what's happening here...

Thanks.

(Django 3.2.9 and Python 3.6.8)