I have heard that to enter the Womb you cannot use seeds, but after you have unlocked the Womb can you use seeds?
Using a seed disables achievements and unlockables for that run. This means that you won't unlock the womb if your first Mom kill is on a seeded run.
You still have access to anything already unlocked though. If you've already beaten Mom, you will be able to continue to the womb after beating her on a seeded run.
I guess you're talking about a bug in an early version of Rebirth on PS4. I don't know exactly how the bug works, but it was possible to unlock Secrets/Achievements and make progress in seeded runs.
However, this bug was fixed months ago on PS4. On PC, it was never possible to unlock things in seeded runs.