I amusing plymouth, systemd and the root partition is encrypted. The initramfs image is generated with dracut.
With all releases of systemd newer than 245.5, the password is not asked and the system hangs and never starts. The splash screen with the box where I should enter the password briefly appears and immediately after that, the text console shows up and is blocked. I can't enter the password to decrypt the root partition. If I remove splash from the kernel command line, I can enter the password in the text console. But with splash enabled, it is not possible. I have tried the 2 newer versions of systemd available currently (245.7-r1 and 246-r1) and the problem occurs with these 2 versions.
You didn't say which version of plymouth you are using, but this looks like a dupe of bug 734938.
*** This bug has been marked as a duplicate of bug 734938 ***
I am using plymouth 0.9.5-r1 and I still have a problem with newer versions of systemd. Therefore, I am not sure it is a duplicate.
I have another computer where the root partition is not encrypted. In this case, there is no hang with plymouth. The problem seems thus to be linked to the prompt for the password.