Correction: I was able to get this to work with v0.88.0 and v0.88.1. Fails with v0.89.0 and later.
When we went from v0.88.1 to v.89.0, we bumped the markdown renderer (github.com/yuin/goldmark) from v1.4.0 to 1.4.2.
Although you may not like to hear it, this worked in v0.88.1 due to a bug in Goldmark. Goldmark adheres to the Commonmark specification. Commonmark provides a demonstration site that adheres to the specification.
When I use their demonstration site with your markup, the output from the current version of Goldmark is an exact match.
When I compare that to the output from v1.4.0 of Goldmark, it is not an exact match. The Goldmark author corrected that somewhere between v1.4.0 and 1.4.2. So your site broke when they fixed the bug.
Going forward, I would use either a shortcode or markdown render hook as described here:
https://discourse.gohugo.io/t/40998/#alternative-3