This bug appears to be related to the feature added in response to this:
https://orionfeedback.org/d/9796-automatically-close-download-url-tabs
Under Mojave at least, this has resulted in the forced closure of tabs when downloading, even when a new tab isn't opened for the download. This is extremely annoying - the tab you are viewing or working on gets closed whenever you do a download. Please ensure this feature ONLY works when a new tab is opened for a download, and only that new download tab is closed. This issue began with 131, and remains with 132.
The tab you are viewing or working on should not be closed when downloading.
132
Mojave (10.14)