This has happened to me on both HW:02 FW:2021.6 as well as HW:03 FW:2021.6… First thing that happens is the program does not recognized unnamed variables, syntax errors, etc. It will still let me download changes and acts as if everything is normal. I cannot create these new variables because it doesn’t seem to think anything is wrong. Then I go live and into debug mode and everything is wonky. The attached image shows a small portion of this. All the new code is messed up, out of order, and doesn’t work. I’d love a fix to this ASAP.
What version of PLCnext Engineer are you using? The developers will probably ask for a copy of a PLCnext Engineer project that shows this error. Are we OK to send you an email (to the address you used to register on this forum) with details of how to send the project?
Did you do a project rebuild before sending? Since updating to 2021.6 I’ve seen some strange behavior with making some changes, and sending changes without a full rebuild.
I have see this before too. It isn’t just Engineer 2021.6 but also has occurred before. As general advice I can only say: keep the „download changes“ small and to an minimum. If you have some more substantial changes - make a full download. You could also see issues with the OPC_UA communication when „download changes“. So again keep the changes small and simple. And yes, I agree - we should not this kind of problem. On the other hand - the PLCNext eco sytem is still relative new and we will see this kind of issues. Frustrating - yes, very! But there are always some workarounds for those issues. No a day goes by, where I don’t want to kick some one from Phoenix for the issues I see, the missing function in Engineer that are standard in other tools (we are in the year 2021 and the world has turned around a few times). But it is a work in progress. On the other hand, I can see the potential for the system and software. That is what keeps me going here. I only hope that we get some proper PID function blocks soon and the integration of IOLink devices gets better (Martin - sorry, could not resist the small kick in your direction).
No problem, I try not to take these things personally. 
What can I say, except that I will (as always) pass on these comments to the responsible people here.