HTTP header cut in half with `urllib3.exceptions.HeaderParsingError: [MissingHeaderBodySeparatorDefect()], unparsed data`

I spotted a weird warning in logs:

[WARNING] urllib3.connectionpool:467: Failed to parse headers (url=https://REDACTED): [MissingHeaderBodySeparatorDefect()], unparsed data: 'trol,Content-Type\r\n\r\n'
Traceback (most recent call last):
File "/usr/local/lib/python3.8/dist-packages/urllib3/connectionpool.py", line 465, in _make_request
assert_header_parsing(httplib_response.msg)
File "/usr/local/lib/python3.8/dist-packages/urllib3/util/response.py", line 91, in assert_header_parsing
raise HeaderParsingError(defects=defects, unparsed_data=unparsed_data)
urllib3.exceptions.HeaderParsingError: [MissingHeaderBodySeparatorDefect()], unparsed data: 'trol,Content-Type\r\n\r\n'

This is from calling a standard requests.post() on a web service I fully control (a Python app behind nginx).

When I turn on debuglevel=1 in http.client.HTTPResponse I see this:

reply: 'HTTP/1.1 200 OK\r\n'
header: Server: nginx/1.18.0 (Ubuntu)
header: Date: Tue, 30 Nov 2021 22:14:04 GMT
header: Content-Type: application/json
header: Transfer-Encoding: chunked
header: Connection: keep-alive
header: Vary: Accept-Encoding
header: Access-Control-Allow-Origin: *
header: Access-Control-Allow-Credentials: true
header: Access-Control-Allow-Methods: GET, POST, OPTIONS
header: Access-Control-Allow-Headers: DNT,X-Mx-ReqToken,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Con

Note the last header ending abruptly in ,If-Modified-Since,Cache-Con.

Clearly, requests==2.26.0 (via urllib3==1.26.7 via http.client) cuts the last header in half for some reason during parsing, and then later complains it has "left over" data with the remaining trol,Content-Type\r\n\r\n.

In this case the warning is not critical, because the header is not really needed. But it's scary this is happening, because… what else is being cut / misparsed?

Any idea what could be wrong? Many thanks.

from HTTP header cut in half with `urllib3.exceptions.HeaderParsingError: [MissingHeaderBodySeparatorDefect()], unparsed data`