Musescore 4: Custom time signature issue

"Is this a bug in Musescore 4"
Probably not as I recall no other reports of this and I can successfully create a 15/16 Time signature that has an actual duration of 15 sixteenths.

"or am I missing something here?"
Probably, but without looking over your shoulder it is impossible to tell what that is.

Here are some pictures of the steps involved.

Step 1 click on the More button in the Time Signature Palette and then click on the Create New Time Signature button

Step 2 Create the new time signature, adjust it beaming properties if necessary and add it to the Time Signature Palette.

Step 3 Find the new time signature in the palette and use it just like any other.

If it doesn't work for you please attach your score (the .mscz file) and describe where the process goes wrong for you.

> Is this a bug in Musescore 4, or am I missing something here?
You are doing the wrong action, you are modifying a single measure length which is intended e.g. for the anacrusis.

You need a custom time signature. Have a look in the time signature palette, press 'More' and then 'Create Time Signature'. It will then be added to the palette and you can use it.