I don't know which OBS version runs on build.opensuse.org
so I selected "2.5" as the latest released one.

What I experienced:

Under obscure conditions 'osc -r revision'
results wrong sources:
Not those of the specified revision
but some older ones (in my case).

What I did:

Some days ago I had accepted
https://build.opensuse.org/request/show/881509
home:dimstar:Factory/ghostscript -> Printing/ghostscript
and
-------------------------------------------------------------------------
# osc log Printing ghostscript | head

r141 | jsmeix | 2021-03-29 08:29:21 | 17214ebb583d93d3fc43297ac02864e2 |
unknown | rq881509

- Do not rely on apparmor at all for the -mini flavor:
  + Drop apparmor-abstraction and apparmor-rpm-macros
    BuildRequires.
  + Do not package apparmor files.
-------------------------------------------------------------------------
confirms that those changes are in Printing/ghostscript.

Today I liked to upgrade Printing/ghostscript
so I did:
-------------------------------------------------------------------------
# osc branch Printing ghostscript

A working copy of the branched package can be checked out with:

osc co home:jsmeix:branches:Printing/ghostscript
Pending requests for package: Printing/ghostscript (1)
881824  State:review     By:dimstar_suse When:2021-03-30T13:01:03
        submit:          Printing/ghostscript@141 ->                       
openSUSE:Factory
        Review by User       is accepted:  licensedigger(licensedigger)         
        Review by User       is accepted:  factory-auto(factory-auto)           
        Review by Group      is accepted:  factory-staging(dimstar_suse)        
        Review by Group      is accepted:  opensuse-review-team(namtrac)        
        Review by Project    is new:      
openSUSE:Factory:Staging:K(dimstar_suse)          
        Comment: Being evaluated by staging project
"openSUSE:Factory:Staging:K"

# osc co home:jsmeix:branches:Printing/ghostscript
...
-------------------------------------------------------------------------
That plain "osc co" without "-r revision" results an older state
from some longer time before rq881509 was accepted.

So I tried
-------------------------------------------------------------------------
# osc branch -r 141 Printing ghostscript

A working copy of the branched package can be checked out with:

osc co home:jsmeix:branches:Printing/ghostscript
Pending requests for package: Printing/ghostscript (1)
881824  State:review     By:dimstar_suse When:2021-03-30T13:01:03
        submit:          Printing/ghostscript@141 ->                       
openSUSE:Factory
        Review by User       is accepted:  licensedigger(licensedigger)         
        Review by User       is accepted:  factory-auto(factory-auto)           
        Review by Group      is accepted:  factory-staging(dimstar_suse)        
        Review by Group      is accepted:  opensuse-review-team(namtrac)        
        Review by Project    is new:      
openSUSE:Factory:Staging:K(dimstar_suse)          
        Comment: Being evaluated by staging project
"openSUSE:Factory:Staging:K" 

# osc co home:jsmeix:branches:Printing/ghostscript
...
-------------------------------------------------------------------------
Again the plain "osc co" without "-r revision" results an older state
from some longer time before rq881509 was accepted
so I tried
-------------------------------------------------------------------------
# osc co -r 141 home:jsmeix:branches:Printing/ghostscript

Revision '141' does not exist
-------------------------------------------------------------------------
which correctly fails because
-------------------------------------------------------------------------
# osc log home:jsmeix:branches:Printing/ghostscript | head

r1 | jsmeix | 2021-04-01 10:00:33 | ad0d78596aba2f83141e5881da1f3614 | unknown
| 
<no message>
-------------------------------------------------------------------------

Finally
------------------------------------------------------------------------ghostscript.changes-
# osc cat -r 141 Printing ghostscript ghostscript.changes | head

Tue Oct 20 16:38:24 CEST 2020 - Ismail D���nmez <idonmez@suse.com>

- 41ef9a0bc36b9db7115fbe9623f989bfb47bbade.patch
  fixes compilation with FreeType 2.10.3+
 
http://git.ghostscript.com/?p=ghostpdl.git;a=commitdiff;h=41ef9a0bc36b9db7115fbe9623f989bfb47bbade
  c.f. https://bugs.ghostscript.com/show_bug.cgi?id=702985
-------------------------------------------------------------------------
proves that under conditions that are obscure to me
'osc -r revision' results wrong sources.

This ghostscript.changes is the above mentioned older state
from some longer time before rq881509 was accepted.

I guess this has something to do with those pending
request 881824 for package Printing/ghostscript
"Printing/ghostscript@141 -> openSUSE:Factory".

But I do not understand why I cannot get
those Printing/ghostscript@141 sources
when I explicitly request them.

I would expect that when I request '-r 141'
then I either get that specified revision
or I get an (error) message why it is not possible.

I think silently providing a wrong revision is a bug.