Could you please show us the value of email header "X-Spam-*" of quarantined mail?
Or, show us the full log line which contains the matched spamassassin rules like this:

(402344-02) Blocked SPAM {DiscardedInbound,Quarantined}, [198.2.179.39]:12643 [54.194.174.44] ESMTP/ESMTP <bounce-md_30790748.68593c30.v1-c22e77decc6348298225da61980c697c@mandrillapp.com> -> <jessica@xxxxx.co.uk>, (ESMTPS://[198.2.179.39]:12643 < 54.194.174.44), quarantine: 5zGMNXJ9VN5L, Queue-ID: 4bQmHq3pvfzbkm0, Message-ID: <30790748.20250623113616.68593c30697270.37773590@mail179-39.suw41.mandrillapp.com>, mail_id: 5zGMNXJ9VN5L, b: 2pzrYiKVQ, Hits: 0.889, size: 46263, Subject: "This week's Insights for your classes (raw: =?utf-8?Q?This=20week's=20Insights=20for=20your=20classes?=)", From: <support@century.tech> (dkim:AUTHOR), helo=mail179-39.suw41.mandrillapp.com, Tests: [DKIMWL_WL_MED=-0.001,DKIM_SIGNED=0.1,DKIM_VALID=-0.1,DKIM_VALID_AU=-0.1,DKIM_VALID_EF=-0.1,HEADER_FROM_DIFFERENT_DOMAINS=0.001,HTML_MESSAGE=0.001,RCVD_IN_DNSWL_BLOCKED=0.001,RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001,RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001,RCVD_IN_ZEN_BLOCKED_OPENDNS=0.001,SPF_HEL...

Pasted one is not full, some text were cut.

As you can see, spam scanning is working, the problem is it doesn't get high score as spam.
Try to append these rules in /etc/mail/spamassassin/local.cf to adjust scores:

#
# Adjust spam scores.
#
score ALL_TRUSTED 0.1

#
# Spamhaus
#
score URIBL_DBL_SPAM 10
score URIBL_DBL_PHISH 10
score URIBL_DBL_MALWARE 10
score URIBL_DBL_BOTNETCC 3
# Contains an abused spamvertized URL listed in the Spamhaus DBL blocklist
score URIBL_DBL_ABUSE_SPAM 10
score URIBL_DBL_ABUSE_REDIR 3
score URIBL_DBL_ABUSE_PHISH 5
score URIBL_DBL_ABUSE_MALW 5
score URIBL_DBL_ABUSE_BOTCC 3
score URIBL_DBL_ERROR 0

# multi.surbl.org
score URIBL_WS_SURBL 10
score URIBL_PH_SURBL 10
score URIBL_MW_SURBL 10
score URIBL_CR_SURBL 10
score URIBL_SC_SURBL 10
score URIBL_OB_SURBL 10
score URIBL_AB_SURBL 10
score URIBL_JP_SURBL 10
score URIBL_ABUSE_SURBL 5
score SURBL_BLOCKED 0

# multi.urlbl.com
#score URIBL_BLACK 10
#score URIBL_GREY 3
#score URIBL_RED 0
score URIBL_BLOCKED 0

# DNSBL
score RCVD_IN_SBL 10
score RCVD_IN_SBL_CSS 10
score RCVD_IN_XBL 10
score RCVD_IN_PBL 10

score RCVD_IN_BL_SPAMCOP_NET 5
score RCVD_IN_PSBL 5
score RCVD_IN_RP_RNBL 5

# SPF
# sender does not match SPF record (fail)
score SPF_FAIL 5

# To == From and direct-to-MX
score TO_EQ_FM_DIRECT_MX 5
# To domain == From domain and HTML image link
score TO_EQ_FM_DOM_HTML_IMG 5
# To domain == From domain and HTML only
score TO_EQ_FM_DOM_HTML_ONLY 5
# To domain == From domain and external SPF failed
score TO_EQ_FM_DOM_SPF_FAIL 5
# To == From and HTML only
score TO_EQ_FM_HTML_ONLY 5
# To == From and external SPF failed
score TO_EQ_FM_SPF_FAIL 5

# Malformed From address
score FROM_ADDR_WS 5

# From: has too many raw illegal characters
score SUBJ_ILLEGAL_CHARS 5

# Link to hosted firebase web application, possible phishing.
#score URI_FIREBASEAPP 5

# Email sent from free email service providers.
# From address is in To and Subject
score FROM_IN_TO_AND_SUBJ 5
# From and body contain different freemails.
score FREEMAIL_REPLY 5

# Many false positives reported by dnswl.org. Disable it here.
score RCVD_IN_DNSWL_HI 0