So, it turns out this wasn't an issue with the MX, but rather an error in each extension within Free PBX:
it seems that for the non-working extensions the “dial” is only the digit 1 instead of the full extension, committing the second and third integer…ext 103 and 112 work and are proper.
In correcting this, all is well
[root@freepbx ~]# asterisk -rx ‘database show DEVICE/100’
/DEVICE/100/default_user : 100
/DEVICE/100/dial : PJSIP/1
/DEVICE/100/tech : pjsip
/DEVICE/100/type : fixed
/DEVICE/100/user : 100
5 results found.
[root@freepbx ~]# asterisk -rx ‘database show DEVICE/103’
/DEVICE/103/default_user : 103
/DEVICE/103/dial : PJSIP/103
/DEVICE/103/tech : pjsip
/DEVICE/103/type : fixed
/DEVICE/103/user : 103
5 results found.
[root@freepbx ~]# asterisk -rx ‘database show DEVICE/102’
/DEVICE/102/default_user : 102
/DEVICE/102/dial : PJSIP/1
/DEVICE/102/tech : pjsip
/DEVICE/102/type : fixed
/DEVICE/102/user : 102
5 results found.
[root@freepbx ~]# asterisk -rx ‘database show DEVICE/112’
/DEVICE/112/default_user : 112
/DEVICE/112/dial : PJSIP/112
/DEVICE/112/tech : pjsip
/DEVICE/112/type : fixed
/DEVICE/112/user : 112
Thanks again for all of your help!