Guess @davidsneighbour is right with his guess regarding summary.
If you use the embedded RSS template there’s been an issue with escaping
I found an issue with HTML escaping solved with v0.121.0
The RSS templates are different:
embedded RSS template for v0.111.2
<description>{{ .Summary | html }}</description>
embedded RSS template for v0.121.0
<description>{{ .Summary | transform.XMLEscape | safeHTML }}</description>
In release v134.0 the behavior of the .Summary changed. (Summary comparision)[Content summaries | Hugo] but – at least regarding summary – nothing has changed in the templates.
embedded RSS template for v0.138.0
<description>{{ .Summary | transform.XMLEscape | safeHTML }}</description>