Aha! 
I have unattended-upgrades disabled. But, I do an apt update/upgrade every day because I use the daily build of FreeCAD. And, having no clue the package for VSC is called “code” it didn’t register to me that it was being upgraded.
Thanks for that! 
Don’t know what was up with the Intellisense thing…it just stopped being weird after an hour or so. 
Yeah, VSCode is a bit sneaky that way… they couldn’t call the package vscode, now could they?!
A few things are happening when the IntelliSense rebuild message is shown… including installing library files, missing toolchains, etc. Maybe it was having trouble doing that.
The same happened to me. Now, i’m here looking for solutions
Thanks. The sudo apt-mark hold code worked for me too (I was also getting the unwanted updates). Well, we’ll see… it is working for now at least 
All the best
thanks @lcerda , from your answer the complete provisional workaround is:
sudo apt-get remove code --purge
sudo apt-get install code="1.39.2-1571154070"
sudo apt-mark hold code
works fine with Debian 10.
Okay, I ran into the same issue today on Windows 10.
I’ve uninstalled VSC, deleted .platformio and .vscode from my user directory. I deleted .vscode and .pio from my project directory. I’ve reinstalled VSC x64 1.40.2 (System setup). Reinstalled PlatformIO, opened my project folder in VSC and… still got the isse 
I’ve then installed VSC on an other Windows 10 PC, copied my project, installed PIO and the tasks worked fine.
I may have also clicked the disable typescript button…
How do I solve the issue? Downgrading is not an option…
Okay, solved the issue:
CTRL+Shift+P: Preferences: Open Settings (JSON)
or
Open %user%\AppData\Roaming\Code\User\settings.json (missed that during reinstall)
Delete "task.autoDetect": "off"
I have no idea how it got there…
Oh god, thank you. You’ve saved me hours of pain, cause my deadline is in two days
Just some more data points.
I tried what @XSRF recommended with CTRL+Shift+P with 1.39
"task.autoDetect": "off"
On a different PC running Ubuntu 19.10, that had no prior install of VSC, I installed VSC 1.40 from scratch.
Build worked fine first try…
just come here to say thank you. saved my night
Thank you!
One more thing, maybe related. At least, disappearing-tasks problem appeared when ms-vscode.cpptools extension self-switched to “insiders” channel (maybe, i clicked “update”, but not sure, anyway - ms-vscode.cpptools want to switch to “insiders”, because when i switched it back to “default” it ask me to swith to “insiders” again, but i have rejected, of course).
And, seems like, this “insiders” “damaged” something else (inluding build tasks of PIO)
Thanks for this hint - saved me lots of pain…
Thankssss!!! saved my life!!
The solution by XSRF worked for for me as well (task.autoDetect was off). Thanks!
it was OK yesterday, today morning, got same issue, but thanks, it works
CTRL+Shift+P: Preferences: Open Settings (JSON)
Delete "task.autoDetect": "off"
This is a very hot topic. I still do not understand. Who does set "task.autoDetect": "off"?
I’m new to platformio, it showed me some message that auto tasks is slowing your system, click here to turn off auto tasks, some notification like that, so I think issue appeared after I clicked that button.
Do you have other VSCode extensions? PlatformIO extension should not slow your machine.
God damn M$… Present people with a “Would you like your computer to stop working?” button and of course they’ll click ‘Yes’ so it goes away! Where was the “If you don’t know what an auto task is, you probably shouldn’t be clicking this button?” warning label? 
Thanks for the solution !!!