I would venture that any legitimate 401(k) plan *does* keep these funds in separate accounts. Why? It's the law.
Here's the excerpt from 26 U.S. Code § 402A (https://www.law.cornell.edu/uscode/text/26/402A)
In my current 401(k) account, the top-level views all show the amounts combined, but if I click down to the "vesting" view, I see the amounts broken out individually. (Note that I don't have any Roth contributions, but my after-tax contributions are treated the same way I expect Roth contributions to be treated)
Here's the excerpt from 26 U.S. Code § 402A (https://www.law.cornell.edu/uscode/text/26/402A)
(2) Separate accounting required
A program shall not be treated as a qualified Roth contribution program unless the applicable retirement plan—
(A) establishes separate accounts (“designated Roth accounts”) for the designated Roth contributions of each employee and any earnings properly allocable to the contributions, and
(B) maintains separate recordkeeping with respect to each account.
In my current 401(k) account, the top-level views all show the amounts combined, but if I click down to the "vesting" view, I see the amounts broken out individually. (Note that I don't have any Roth contributions, but my after-tax contributions are treated the same way I expect Roth contributions to be treated)