You know that "one of the..." as a subject is always singular since we are talking about ONLY ONE entity.

However, when a relative pronoun clause is placed next to it, the decision of using singular or plural verb depends on the context of the sentence.

1: If the relative pronoun clause describes all the entities, then plural verb should be used.
2: If it describes just the ONE of the many entities, then singular verb should be used.

So in essence, it is all context driven...
Lets consider some examples:

1A: One of the apples was rotten. (one of the...as subject)
Here "one of the .." is the subject. And hence singular. From the context as well, we are emphasizing that only 1 apple was rotten - thus singular verb was.

1B: One of the apples that were rotten was mistakenly used in the pudding.
Notice here: We are saying that "Only 1 rotten apple" was used in the pudding.
But we are also saying that all apples were rotten.

1C: The only one apple that was rotten was mistakenly used in the pudding.
Again only 1 rotten apple was used in the pudding.
But now the sentence implies that this was the only 1 rotten apple.

So as you can see when relative pronoun clauses are used after the phrase "one of the noun", selection of plural vs. singular verb depends on the meaning of the sentence. So it is very essential to understand what the sentence implies.

Lets take another set of examples:

2A: Dhoni is one of those cricketers who do modeling assignments.
Lets simplify this sentence:

Dhoni is one of those cricketers
who do modeling assignments.

This sentence implies that many cricketers do modeling assignments and Dhoni is just one of them.
"who" refers to "cricketers" and hence is plural. Thus the plural verb -do.

2B: Dhoni is the only one of those cricketers who does modeling assignments.

In this sentence the context has changed. Now the author of the sentence is stating the fact that Dhoni is the only cricketer who does modeling assignments. So as you can see the word 'only' changes the emphasis in the sentence.

So as I always say, understand the meaning of the sentence and then select correct grammar to use to express that meaning.

A higher interest rate is only one of the factors, albeit an important one, that keeps the housing market from spiraling out of control, like it did earlier in the decade.

The intended meaning:
Although it is an important factor, higher interest rate is ONLY 1 of the many factors. All these factors keep the housing market from spiraling out of control. And higher interest rate acted in the same manner earlier in the decade as well.
Thus, in this sentence the relative pronoun clause "that keeps..." modifies ALL the FACTORS and hence should be "that keep..."

Another version of the sentence:
A higher interest rate is the only one of the factors that keeps the housing market from spiraling out of control, like it did earlier in the decade.

The intended meaning:
Higher interest rate is THE ONLY 1 of the many factors. This 1 factor keeps the housing market from spiraling out of control. And higher interest rate acted in the same manner earlier in the decade as well.
Thus, in this sentence the relative pronoun clause "that keeps..." modifies ONLY higher interest rate or ONLY 1 of the factors and hence should be "that keeps..."

Bottomline: Understand the intended meaning of the sentence and decide accordingly.

Note that grammatically "that" cannot refer to "an important one". Notice carefully the sentence structure:
A higher interest rate is only of the factors, extra information, that keep the housing market from spiraling out of control...
"that modifier" is directly linked to the main clause. And remember, the modified entity and "that clause" are never separated by a comma. This is because "that clause" always provides essential information about the modified entity.
Check out the link below for more information about this:
https://e-gmat.com/blogs/?p=575

Thanks,

Payal