How to stop # bringing up Github issues list?

Thanks for the tips, folks - it got me going down the right route by checking what was happening for different filetypes and double-checking installed extensions.

Eventually I found the following under the GitHub plugin (which I thought was a core feature of VSC)...

Github Issues: Ignore Completion Trigger Languages that the '#' character should not be used to trigger issue completion suggestions.

It had python and makefile already added. I tried adding Shell Script and plain text but it didn't seem to make a difference. However I then found Github Issues › Issue Completions and was able to turn it off completely with that. It means it doesn't appear when doing the commit message either, but that's less annoying than the previous behaviour when I was trying to add comments to my shell script and kept getting these popups!