"Modifications Aren't in Synch" - VPN problem?

Quite often now (OS : latest Sonoma, 2022 Macbook Pro), I am in of the middle of updating a spreadsheet (Numbers, latest version) when there's a pop-up of doom which I can't ignore saying "Modifications aren't in Synch". You have to take a quick look, and generally select the upper of two choices, before you can proceed.

Yup. But this is happening every 30 seconds or so, while writing into the same spreadsheet. Why doesn't the system know I'm currently editing the spreadsheet, and wait a while until I've finished - or at least wait 15 minutes or so?

Could it be my use of a VPN?

To try to cure this, I have switched off the WiFi at the icon at the top of the screen. Shockingly, it keeps happening, and I'm astonished that I'm able to access the internet at all, with the WiFi set to Off. That's a puzzle too, if you'd care to comment.

Any (really simple) advice welcome - best wishes to you,

Tim