Using ADDPFM in QCMDEXE via RPG

Sorry for the lengthiness of the post but wanted to elaborate on the issue
and all the steps taken so as not to get questions back for things we have
already tried.

We have a multi-member file in a library that is shared across multiple
library lists in a multi-tenant LPAR. Jobs often run the same
RPGLE program at the same time whose function is to add a new member to the
shared file. This process is run in a client-specific single threaded job
queue so the program can run concurrently between "clients". It has been
performing amicably in this batch process for many years without issue.

We are at 7.4 O/S and have installed PTFs in March for a QTEMP cataloguing
issue but the file in question, here, is not a QTEMP file.

The number of members in the subject file is monitored daily to remove very
old members so that we do not exceed the max member limit.

Recently, several jobs threw an MCH5802 escape message (Lock operation for
object &1 not satisfied) from program RmslLockMiObject which culminated in
a CPF7306 escape message (Member &1 not added to file &2 in &3.).

A monitor-endmon block was added to the RPG program but we have been unable
to test and recreate the error even when we issue an ALCOBJ command with
lock state *EXCL on a second workstation. We have also tried using CHGPF
and RGZPFM to create a lock on the file, yet the ADDPFM command in the RPG
still creates a member (does not throw the lock error).

FYI, when we try to use DBU on that file, after the ALCOBJ command, it
throws (CPF3203) "Cannot allocate object for file &1 in &2." Also, when we
created a CLLE program to be called from the RPGLE program to perform the
ADDPFM statement, it still creates the member in the subject file and does
not throw any errors.

Does anyone have any advice on how to get a lock on the file that will
create a CPF7306 error?