Actually, I'm almost certain that that's what happened now.  (That it likely chose the default_wallet.dat file from the Roaming directory when it was, this morning, in the Program Files/Electrum directory.)

This morning, I ran my usual bash script:

cd /cygdrive/c/Program\ Files\ \(x86\)/Electrum/
./electrum & ./electrum -w seedless-orig.dat

Until I'd installed 2.7.18, this would load a default_wallet.dat hot wallet from the Program Files/Electrum directory and my cold storage wallet from that same directory.  

Both of those files disappeared this morning from that directory.  There are now no longer any wallet files in:

C:\Program Files (x86)\Electrum

So, Electrum actually deleted two wallet files of mine, this morning.  I saw them after the installation in the folder when I started searching.  One minute they were there, the next they were gone.  

EDIT: Just out of curiosity, I just did a search over my hard drive for 'seedless-orig.dat'.  That was one of the wallets tha I claimed was deleted.  It now seems like it was moved to a series, or what looks like 9 locations, similar to this:

C:\Users\btcbot\AppData\Local\Application Data\Application Data\Application Data\Application Data\Application Data\VirtualStore\Program Files (x86)\Electrum

I found 9 copies, modified from the last time I opened it.  I don't know if these are just Windows soft-links or what. 

A similar search over my hard drive for 'default_wallet.dat' produced nothing.