OK, when I added the same debug messages to ox-html.el, I see:

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(table (:begin 1860 :end 2104 :type table\.el :tblfm nil :contents-begin nil :contents-end nil :value "+----------+----------+----------+
| Header 1 | Header 2 | Header 3 |
+----------+----------+----------+
| a        | b        | c        |
+----------+----------+----------+
| d        | e        | f        |
+----------+----------+----------+" :post-blank 1 :post-affiliated 1860 :parent (section (:begin 1703 :end 2104 :contents-begin 1703 :contents-end 2104 :post-blank 1 :post-affiliated 1703 :parent (org-data nil #2)) (special-block (:type "description" :begin 1703 :end 1783 :contents-begin 1723 :contents-end 1765 :post-blank 1 :post-affiliated 1703 :parent #2) (paragraph (:begin 1723 :end 1765 :contents-begin 1723 :contents-end 1765 :post-blank 1 :post-affiliated 1723 :parent #3) #("Support tables written in table.el format
" 0 42 (:parent #4)))) (paragraph (:begin 1783 :end 1860 :contents-begin 1783 :contents-end 1859 :post-blank 1 :post-affiliated 1783 :parent #2) (verbatim (:value "ox-hugo" :begin 1783 :end 1793 :post-blank 1 :parent #3)) #("Issue #" 0 7 (:parent #3)) (link (:type "https" :path "//github.com/kaushalmodi/ox-hugo/issues/374" :format bracket :raw-link "https://github.com/kaushalmodi/ox-hugo/issues/374" :application nil :search-option nil :begin 1800 :end 1858 :contents-begin 1853 :contents-end 1856 :post-blank 0 :parent #3) #("374" 0 3 (:parent #4))) #("
" 0 1 (:parent #3))) #0)))
"<!-- This HTML table template is generated by emacs 28.0.50 -->

=====

ox-hugo is stripping the leading/trailing spaces from the table cells, while ox-html is not. Somehow that doesn't play with the recent change to table.el detection.

I'll try fixing that in ox-hugo.