Tony,
I am using essentially an original TW5 file. I have not added any plug-ins or modified any system tiddlers (I am pretty much a newbie on this version). The only thing I use aside from the original html is TiddlyWiki In the Sky (TWITS) -- The Dropbox connector for TW. As its somehow involved with the saving could it be the culprit?
I might add that I am using Firefox 74.0 and just now tested Chrome 80.0.3987.149 and the problem does not occur on it. I am suspecting that it is a problem with TWITS and FF. Will post a question over at the TWITS support site and update here once I get a response.
2020-03-23 update
After numerous testing activities I am pretty much convinced that this is a TWITS-Dropbox issue. If I get a resopnse from the TWITS support site I will cross post here.
-->Michael