Keep in mind... this can (and probably will) still happen with FP enforcement. In fact, it's very likely that tour group was not using FPs late but DURING their window.
At any given time (let's use 12:30pm as an example), everyone with a valid FP COULD all show up at the same time. It's not just people with a return time that STARTS at 12:30pm. This would include everyone with FP return times of:
11:30am-12:30pm
11:35am-12:35pm
11:40am-12:40pm
11:45am-12:45pm
11:50am-12:50pm
11:55am-12:55pm
12:00pm-1:00pm
12:05pm-1:05pm
12:10pm-1:10pm
12:15pm-1:15pm
12:20pm-1:20pm
12:25pm-1:25pm
12:30pm-1:30pm
So you can see that at ANY given time a LOT of people could show up and not be late (anyone happen to know how many FPs are distributed before the return time clock moves ahead 5 minutes?). If there happen to be tour groups at the park, the odds increase of a lot of people showing up all at once. But, if you just happen to have bad luck, lots of individual families could all show up at once by coincidence.
Regardless of the reason the posted wait time increased, I can understand being frustrated because you got into the line on good faith, expecting to be done in time to make your next FP window. If there are complaints, these are the kinds of circumstances most likely to elicit them. It will be difficult for CMs to make a judgment call if a guest is telling them the sign said 30 minutes, but I waited an hour and a half!