This warning is new in v6, the issue has most likely been there before, you just never noticed. Most (if not everyone) is having intermittent Internet outages (or just badly chose routes, they change all the time!) and packets get lost. Usually, the web servers are using TCP which has an in-built retransmission mechanism for exactly this usecase, UDP just doesn't have it. You probably still don't know about this as the original requestor will just re-query after a timeout of maybe two seconds and this time it will work.
Hence, a question for you guys: Looking at /var/log/pihole/FTL.log - how often do you see this? If it happens only a few times an hour, it is nothing to be worried about at all. If it happens like thousands of times per minute, this is more real of an issue.
I think we somehow need to make this warning less obvious to avoid users reading to much into it. I was thinking about maybe a threshold, only putting up the warning if we have seen more than, say, 10 of them in the last minute. Any objections/suggestions? Picking this threshold also depends on your feedback concerning the frequency in your log files.