Does the problem still occur when you install the TC version of Topic List Previews and uninstall the plugin? That only uses the core generation logic and does not change anything in that regard.
If so it’s not TLP that’s the issue.
See this code which details the inclusion criteria:
which is slightly broader than core (plugin only, TLP TC uses core logic so long as you don’t install the sidecar plugin). As you can see it includes any img with a src attribute except those that fit the exclusion criteria. This code is identical to core bar the end.
Please remove the plugin, install the TC, and confirm if the problem persists or not.