Have you checked the md5sum on gawk executable, compared with that on other machines? Since you re-installed, I'm just clutching at straws.
IMO, you really need to figure out why this is happening. We all know from many years of experience that there is no "magic". There is always an explanation, and some funky behavior like this has to be diagnosed and fixed. There is something weird going on, awk printing lines without a match. Logically, either the input file is changing (but would not explain awk reporting non-matching lines), or awk is damaged (seems impossible since you re-installed), or a hardware problem (?), or something else (no guess). I hope that machine is not doing anything mission-critical.
If you have 100+ machines, and awk continues to malfunction, maybe this machine should be sold off?
What happens if you copy /etc/fstab to your user directory, and run awk as the user instead of as root? Or vice versa? Again, just clutching at straws.