Sequence name with capital letters issue

Hello,

I'm using a PostgreSQL database.
PostgreSQL 10.17 on x86_64-redhat-linux-gnu

I have some sequences called like that :
app_user_SEQ
app_address_SEQ
...

The issue is that when I call the next value of the sequence, I receive an
error.

When I try :

SELECT nextval("app_user_SEQ");

I have the following error :

SQL Error [42703]: ERROR: column "app_user_SEQ' does not exist

I don't understand why ? So I try to rename my sequences by writing them in
lowercase.
So now my sequences are like that :
app_user_seq
app_address_seq
...

And now the SELECT nextval("app_user_seq"); is working.

I also tried to add again a sequence with a part in capital letters and I
kept the lowercase one in the DB. Now I have these 4 sequences :
app_user_SEQ
app_address_SEQ
app_user_seq
app_address_seq

But now, something interesting happened.
When I call this query : SELECT nextval("app_user_SEQ"); , it's the
app_user_seq sequence that is incremented and not the app_user_SEQ.

I didn't find in the documentation something that mentioned this issue
regarding sequence names with capital letters.
Is it a bug ? or is it something that I didn't find in the documentation ?

Thank you in advance