POSIX says:
SHELL This variable shall represent a pathname of the user's
preferred command language interpreter. If this interpreter
does not conform to the Shell Command Language in XCU
Chapter 2 (on page 2345), utilities may behave differently
from those described in POSIX.1-2017.
There is no requirement to match one of the /etc/shells pathnames.
I have hopefully good news for you! You can set the SHELL variable yourself to your preferred value. For example:
(The details might vary depending on the shell you are currently in.) usrmerge does not affect this at all.