I have long been confused about this relative phrase. Rules, and also experiences from most questions tell me "that is/that are" is wrong, redundant, wordy, not concise, majority from OG's explanation.

Here are some contradictory examples:
1)
OG13-7, option B and C
"that are" is wrong here.
The same error applied to OG13-28A, OG13-43-C, OG13-60-E(that was), OG13-72-E, OG13-82-C, OG13-83-D, OG13-86-C, OG13-106-E

OK, it seems a perfect rules although there is little tricky on OG13-68.

2)
When I applied the rules to OG13-68, option E, it seems worked at first:
OG said "plan itself cannot do the action of "cut"".
OK. "that are" is wrong here again.
But actually in E, it's "that are to cut", so "plans are to", plans' object is to do sth, why wrong?

By the way, OG16-28 in D/E, "that are" also wrong.

Then, let's look at something else:
3)
Did we just mentioned OG13-86 above? Yes, if we looked at the correct answer OG13-86-B, "a phenomenon THAT IS explained" turned out to be a correct answer!

More importantly,
4) OG13-96-D
"a body of work THAT WAS rooted" is right again!

5) OG16-52-original-sentence
"50 drugs that were advertised" is obviously right since it's not underlined.

6) OG16-137-A
"17 nearby stars that"

I don't know how to understand this contradictory.

Thanks in advance.