newvim :setlocal buftype=acwrite & E676

[JESii]

I've been using this plugin for ages and love it.

Recently, however, it's now setting local valuebuftype=acwrite when I open a saved session which leads to an E676 when I try to save the file. I switched to using neovim in the past several weeks, after trying coc:nvim with no problems.
Once the buftyype=acwrite is set, it sticks to the file, even if I open the file without sessions.

TEst sequence

• Open file - buftype=""
• Save file
• Reopen file - buftype=""
• Save session - buftype=""
• Reopen session = buftype="acwrite"
• Reopen file = buftype=acwrite"
• :setlocal buftype=""
• Save file
• Reopen file - buftype=""
• Reopen session = buftype=acwrite"

There is at least one file for which this doesn't happen; ls -la appears similar for both files. So far, OpenSession is the only thing I'm experiencing that sets this invalid buftype="acwrite" value. I checked the MacOS extended attributes and both files have no other attributes.

I'm on Mac BigSur
nvim - 0.5.1, compiled via homebrew

I retested this with vim and do not have that problem.

Any suggestions on what I should look for to resolve this?