I can confirm this doesn't work when routing a call from an inbound route -> did -> ring group -> extension/user -> fmfm list -> external number w/confirmation. Calls just drop before the call can be handled at the remote number. However, if you bypass the ring group on the inbound route and go direct to the extension in the sequence and call confirmation works fine. Put the ring group back as the destination on the inbound route, and disable call confirmation, all works fine. ...But trying to use a ring group with FMFM enabled on an extension with external numbers which is part of a ring group, and now call confirmation doesn't work. As a matter of fact the FreePBX configs go haywire and can start doing all kinds of crazy things like dialing the remote number(s) in the FMFM list 5 times and then giving up. I have even received voicemails from the system recording the call confirmation prompts minutes after both parties have disconnected the call. It is a mess. Perhaps it just wasn't intended to work this way, but whatever, IMHO this is broken badly. I have not googled or checked to see if there is any kind of ticket open on this issue, but I did confirm it is an issue - at least reproducible on my system.
In this case, the workaround might be to route the call to a virtual or real extension with FMFM enabled and include a ring group in the FMFM list of numbers to call instead of routing the number straight to the ring group first? Don't know if that would work in your situation or not?
FreePBX 13.0.42
Asterisk 11.19.0
FreePBX Distro Update version 6.12.65-31