The problem here seems to be that the files are already downloaded, and when asking sourceforge to continue the download, it does the redirect rather than responding with a 416 error with the appropriate content-range as it should.

jak@jak-t480s:$ /usr/lib/apt/apt-helper -q -o debug::acquire::http=1 -o debug::acquire::https=1 download-file https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm cabextract-0.6-1.src.rpm
GET /project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm HTTP/1.1
Host: newcontinuum.dl.sourceforge.net
User-Agent: Debian APT-HTTP/1.3 (1.8.0)


Answer for: https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm
HTTP/1.1 200 OK
Date: Mon, 29 Apr 2019 18:52:51 GMT
Server: Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips
Last-Modified: Thu, 15 Aug 2002 15:45:22 GMT
ETag: "172cf-3a841fe3ec880"
Accept-Ranges: bytes
Content-Length: 94927
Connection: close
Content-Type: application/octet-stream

Get:1 https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm [94,9 kB]
Fetched 94,9 kB in 2s (55,5 kB/s)
jak@jak-t480s:$ /usr/lib/apt/apt-helper -q -o debug::acquire::http=1 -o debug::acquire::https=1 download-file https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm cabextract-0.6-1.src.rpm
GET /project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm HTTP/1.1
Host: newcontinuum.dl.sourceforge.net
Range: bytes=94927-
If-Range: Thu, 15 Aug 2002 15:45:22 GMT
User-Agent: Debian APT-HTTP/1.3 (1.8.0)


Answer for: https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm
HTTP/1.1 302 Found
Date: Mon, 29 Apr 2019 18:52:54 GMT
Server: Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips
Location: http://downloads.sourceforge.net/mirrorproblem?failedmirror=newcontinuum.dl.sourceforge.net
Connection: close
Transfer-Encoding: chunked
Content-Type: text/html; charset=iso-8859-1

Err:1 https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm
  Redirection from https to 'http://downloads.sourceforge.net/mirrorproblem?failedmirror=newcontinuum.dl.sourceforge.net' is forbidden [IP: 64.79.96.4 443]
E: Failed to fetch https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm  Redirection from https to 'http://downloads.sourceforge.net/mirrorproblem?failedmirror=newcontinuum.dl.sourceforge.net' is forbidden [IP: 64.79.96.4 443]
E: Download Failed
jak@jak-t480s:$ rm cabextract-0.6-1.src.rpm 
This tool has been deprecated, use 'gio trash' instead.
See 'gio help trash' for more info.

jak@jak-t480s:$ /usr/lib/apt/apt-helper -q -o debug::acquire::http=1 -o debug::acquire::https=1 download-file https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm cabextract-0.6-1.src.rpm
GET /project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm HTTP/1.1
Host: newcontinuum.dl.sourceforge.net
User-Agent: Debian APT-HTTP/1.3 (1.8.0)


Answer for: https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm
HTTP/1.1 200 OK
Date: Mon, 29 Apr 2019 18:52:57 GMT
Server: Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips
Last-Modified: Thu, 15 Aug 2002 15:45:22 GMT
ETag: "172cf-3a841fe3ec880"
Accept-Ranges: bytes
Content-Length: 94927
Connection: close
Content-Type: application/octet-stream

Get:1 https://newcontinuum.dl.sourceforge.net/project/corefonts/cabextract/0.6/cabextract-0.6-1.src.rpm [94,9 kB]
Fetched 94,9 kB in 2s (55,9 kB/s)