Version:
curl --version
curl 7.88.1 (x86_64-suse-linux-gnu) libcurl/7.88.1 OpenSSL/3.0.8 zlib/1.2.13 brotli/1.0.9 zstd/1.5.2 libidn2/2.3.3 libpsl/0.21.1 (+libidn2/2.3.3) libssh/0.10.4/openssl/zlib nghttp2/1.50.0
Release-Date: 2023-02-20
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS brotli GSS-API HSTS HTTP2 HTTPS-proxy IPv6 Kerberos Largefile libz NTLM NTLM_WB PSL SPNEGO SSL threadsafe TLS-SRP UnixSockets zstd
---------------------
Here is the cookies.txt file before:
# Netscape HTTP Cookie File
# https://curl.se/docs/http-cookies.html
# This file was generated by libcurl! Edit at your own risk.

127.0.0.2	FALSE	/func_test	FALSE	1709598616	mycookie6	991
#HttpOnly_127.0.0.2	FALSE	/func_test	FALSE	1709598616	mycookie5	990
#HttpOnly_127.0.0.2	FALSE	/func_test	FALSE	1709598616	mycookie4	950
#HttpOnly_127.0.0.2	FALSE	/func_test	FALSE	1709598616	mycookie3	900
#HttpOnly_127.0.0.2	FALSE	/func_test/	FALSE	1709598616	mycookie2	5900
#HttpOnly_127.0.0.2	FALSE	/	FALSE	1709598616	mycookie1	4900
#HttpOnly_127.0.0.2	FALSE	/func_test/	FALSE	0	mycookie	1200
****
---------------------
The following is the output from:
curl -v -b cookies.txt -c cookies.txt "http://127.0.0.1/func_test/del_cookie"
---------------------
*   Trying 127.0.0.2:80...
* Connected to 127.0.0.2 (127.0.0.2) port 80 (#0)
> GET /func_test/del_cookie HTTP/1.1
> Host: 127.0.0.2
> User-Agent: curl/7.88.1
> Accept: */*
> Cookie: mycookie2=5900; mycookie=1200; mycookie3=900; mycookie4=950; mycookie5=990; mycookie6=991; mycookie1=4900
> 
< HTTP/1.1 200 OK
< Date: Sun, 05 Mar 2023 00:30:22 GMT
< Server: Apache
< Cache-Control: max-age=0, no-cache
< Pragma: no-cache
* Added cookie mycookie2="" for domain 127.0.0.2, path /, expire 1
< Set-Cookie: mycookie2=; Path=/; Max-Age=-1; Expires=Thu, 01 Jan 1970 00:00:00 GMT
* Replaced cookie mycookie="" for domain 127.0.0.2, path /func_test/, expire 1
< Set-Cookie: mycookie=; Max-Age=-1; Expires=Thu, 01 Jan 1970 00:00:00 GMT
* Replaced cookie mycookie3="" for domain 127.0.0.2, path /func_test, expire 1
< Set-Cookie: mycookie3=; Path=/func_test; Max-Age=-1; Expires=Thu, 01 Jan 1970 00:00:00 GMT
* Replaced cookie mycookie4="" for domain 127.0.0.2, path /func_test, expire 1
< Set-Cookie: mycookie4=; Path=/func_test; Max-Age=-1; Expires=Thu, 01 Jan 1970 00:00:00 GMT
* Replaced cookie mycookie5="" for domain 127.0.0.2, path /func_test, expire 1
< Set-Cookie: mycookie5=; Path=/func_test; Max-Age=-1; Expires=Thu, 01 Jan 1970 00:00:00 GMT
* Replaced cookie mycookie6="" for domain 127.0.0.2, path /func_test, expire 1
< Set-Cookie: mycookie6=; Path=/func_test; Max-Age=-1; Expires=Thu, 01 Jan 1970 00:00:00 GMT
* Replaced cookie mycookie1="" for domain 127.0.0.2, path /, expire 1
< Set-Cookie: mycookie1=; Path=/; Max-Age=-1; Expires=Thu, 01 Jan 1970 00:00:00 GMT
< Transfer-Encoding: chunked
< Content-Type: text/html;charset=utf-8
< 
---------------------
However, the cookies.txt file has NOT changed most of the cookies (i.e. evicted the above where expired, mycookie, mycookie3, mycookie4, mycooki5 and mycookie6 should have been removed) - here is the cookies.txt after the above:
---------------------
# Netscape HTTP Cookie File
# https://curl.se/docs/http-cookies.html
# This file was generated by libcurl! Edit at your own risk.

#HttpOnly_127.0.0.2	FALSE	/func_test/	FALSE	0	mycookie	1200
#HttpOnly_127.0.0.2	FALSE	/func_test	FALSE	1709598616	mycookie3	900
#HttpOnly_127.0.0.2	FALSE	/func_test	FALSE	1709598616	mycookie4	950
#HttpOnly_127.0.0.2	FALSE	/func_test	FALSE	1709598616	mycookie5	990
127.0.0.2	FALSE	/func_test	FALSE	1709598616	mycookie6	991
#HttpOnly_127.0.0.2	FALSE	/func_test/	FALSE	1709598616	mycookie2	5900
---------------------
You can see that only mycookie1 with path of "/" was evicted, all others stayed.
---------------------
This behavior is only in 7.88.1. It was NOT there in 7.81.0, where all cookies are deleted properly.