You are using Unicode. On my 8 bit install it stops after 255.

for i=250:1:260 { write i," ", $c(i),! }
250 ú
251 û
252 ü
253 ý
254 þ
255 ÿ
256
257
258
259
260

8 bit or 16 bit storage option on install - https://docs.intersystems.com/iris20252/csp/docbook/DocBook.UI.Page.cls…
Different character set for the locale - https://docs.intersystems.com/iris20252/csp/docbook/DocBook.UI.Pag
e.cls?KEY=GORIENT_localization#GORIENT_localization_locales or Configuring National Language Support (NLS)

Could even be different character sets in the terminal settings to the linux servers.

As a guess the routine is returning 1 and you can demo this using @Dmitry Maslennikov suggestion:

run do $zu(4,$job,1) and check errorlevel

run do $zu(4,$job,0) and check errorlevel

With a mirror you can use iris qlist to detect the primary and not run there. It is a great idea to snapshot the failover backup/DR async as there can not be a user impact that way. 
I would look to SAN devices that offer deduped and compressed and immutable snapshots. Many storage vendors can do this now. The WRC might have reference designs and you should ask them.