Found the right configuration with the help of the Squid Users mailing list.
I had to add different options to ignore cache control and force the cache to keep and serve the content.
But it's working now.
For the record, I'm posting the working Squid Configuration below.
http_port 10.10.10.10.108:3128
icp_port 0
digest_generation off
dns_v4_first on
pid_filename /var/run/squid/squid.pid
cache_effective_user squid
cache_effective_group proxy
error_default_language en
icon_directory /usr/local/etc/squid/icons
visible_hostname pfSense Firewall
cache_mgr pfsense@virtualdesk.cloud
access_log /var/squid/logs/access.log
cache_log /var/squid/logs/cache.log
cache_store_log none
netdb_filename /var/squid/logs/netdb.state
pinger_enable on
pinger_program /usr/local/libexec/squid/pinger
logfile_rotate 7
debug_options rotate=7
shutdown_lifetime 3 seconds
forwarded_for on
uri_whitespace strip
refresh_pattern -i \.(jpg|gif|png|txt|docx|xlsx|pdf) 30240 100% 43800 override-expire ignore-private ignore-reload store-stale
cache_mem 128 MB
maximum_object_size_in_memory 20480 KB
memory_replacement_policy lru
cache_replacement_policy lru
minimum_object_size 0 KB
maximum_object_size 50 MB
cache_dir ufs /var/squid/cache 100 16 256
offline_mode on
cache_swap_low 90
cache_swap_high 95
cache allow all
# Add any of your own refresh_pattern entries above these.
refresh_pattern ^ftp: 1440 20% 10080
refresh_pattern ^gopher: 1440 0% 1440
refresh_pattern -i (/cgi-bin/|\?) 0 0% 0
refresh_pattern . 0 20% 4320
#ACL allow all
acl allsrc src all
http_access allow allsrc
request_body_max_size 0 KB
delay_pools 1
delay_class 1 2
delay_parameters 1 -1/-1 -1/-1
delay_initial_bucket_level 100
delay_access 1 allow allsrc
# Reverse Proxy settings
https_port 10.10.10.10.108:443 accel cert=/usr/local/etc/squid/599eae0080989.crt key=/usr/local/etc/squid/599eae0080989.key defaultsite=tenant.sharepoint.com vhost
#
cache_peer 13.107.6.151 parent 443 0 ignore-cc no-query no-digest originserver login=PASSTHRU connection-auth=on round-robin ssl sslflags=DONT_VERIFY_PEER front-end-https=auto name=rvp_sharepoint