"Stoic" as an adjective and "stoical"

In common speech, I often hear stoic used as an adjective. (I am ignoring definitions related to Stoicism.)

Merriam-Webster validates this use with the definition "not affected by or showing passion or feeling." It also gives stoical as a variant.

Meanwhile, Oxford only calls the adjective "another form for stoical." It defines stoical as "enduring pain and hardship without showing one's feelings or complaining," which matches Merriam-Webster's definition of stoic.

I understand words can have multiple accepted variations of spelling, but the difference between stoic and stoical seems more than just spelling.

If these two major dictionaries disagree about which form is more standard, which one actually is? The idea that stoic is only a noun and stoical its adjective form, as suggested by the Oxford dictionary, makes sense but conflicts with common usage. If the common choice of stoic, supported by Merriam-Webster, is correct, why does the Oxford dictionary defer it to stoical?

Perhaps this is simply an issue of personal preference, but I am at least curious about reasons one form may be preferred over the other.

Stoic is a commonly accepted alternative spelling to stoical. "The adjective (stoic) is recorded from 1590s in the "repressing feelings" sense, c. 1600 in the philosophical sense". (Etymonline)

• of or relating to the school of philosophy founded by Zeno, who taught that people should be free from passion, unmoved by joy or grief, and submit without complaint to unavoidable necessity.

• (lowercase) stoical.

(Dictionary.com)

The following Google Books search appears to suggest that the adjective stoic is becoming more common than stoical.