I have fiddled around with this more today. It looks like it has to do with the language that is chosen. I was honestly very confused by this. It wasn’t obvious at all what was going on. This customer was set at ‘default language’ as far as I can tell. I may have started this invoice before I set the system language, so there may have been an awkward sequencing here.
Maybe something picked up on the fact that I was using euros and went on the basis of that. Many euro countries do indeed use periods for thousands. But Ireland and maybe a few others do not. I think it is all in CLDR - Unicode Common Locale Data Repository
Again, this issue. The standard locale tables for the country seem to indicate the use of a comma as a decimal separator for this country.
I decided to leave it and send the invoice with that formatting.
But for me it is kind of weird that my invoice format varies depends on the country I am sending to. There is a logic to it, sure, but somehow it doesn’t feel quite right!