49% is significant, as it's a deviation of 16 points from what you'd expect. Maine's 28% is not nearly as significant, as it's only 5 points lower than would be expected. Given that these are percentages it's hard to make any significance out of it; even a team that gives up a greater proportion of its goals in the third period can come from behind to win at the end if they also score more in the third period than average, and/or if they allow so few goals on average that the increased likelihood of giving up a late goal makes little difference.
Yes, that's true-- statistically, Maine's defense and goaltending are not significantly worse in the 3rd period compared to any other period. However, if you give up a fair number of goals and you only have a one or two goal lead, you can lose that lead and the game while playing a thoroughly average period.
Maine has given up 65 goals in 23 league contests, which is an average of .94 goals per period. So that means sometimes you'll give up one, but sometimes you'll give up two and other times you'll give up none. Two goals is enough to have a one-goal lead turn into a one-goal loss, and that can happen in 7.6 games over the course of the season without deviating at all from the expected results. (Incidentally, Maine has 8 league losses, so they could have given up a third period lead in every loss this season and it wouldn't necessarily indicate anything statistically significant about 3rd period defense, but just an inevitable result of how many you score and how many they score in an average game, regardless of when.)
UNH has given up only 46 goals in those same 23 contests. That's a 2.0 goals against average, even, which means .66 goals per period. That means they give up one goal per period only slightly more often than they give up none. Since they only give up, on average, two goals per game, any period in which they give up a goal has given up 50% of the goals allotted for that game. In short, UNH gives up so few goals that even the 49% given up in the 2nd period may not be much more than statistical noise. The teams that have given up fewer goals will have less statistical significance attached to any per-period trend because the size of the pool is smaller.
I think that 7 blown leads would certainly lead to a subjective opinion that a team has a bad 3rd period defense and can't hold a lead, without that being a valid statistical judgment. We also haven't taken offense into account at all; it can be just as much a failure to maintain a lead as it is giving a lead up, because on average you'd expect your opponent to have their scoring equally distributed as well, and if they average 3 goals per game then odds are they will score one in the third.
Maine is scoring 3.17 goals per game, which is 1.05 goals per period, so the margin is pretty thin. All other things being equal you'd expect them to win a few more games than lose, and win a few more periods than they lose, and of those losses, some would be 3rd period leads lost.