No issues with queues and 15 here:
freepbx*CLI> core show version
Asterisk 15.0.0 built by mockbuild @ jenkins7 on a x86_64 running Linux on 2017-10-03 13:44:17 UTC
freepbx*CLI> queue show 700
700 has 1 calls (max unlimited) in 'ringall' strategy (2s holdtime, 3s talktime), W:0, C:1, A:0, SL:100.0%, SL2:100.0% within 60s
5013 Aastra (Local/5013@from-queue/n from hint:5013@ext-local) (ringinuse enabled) (dynamic) (Ringing) has taken 1 calls (last was 43 secs ago)
1. PJSIP/5014-0000002d (wait: 0:05, prio: 0)
The only time I have seen (Invalid) for an extension in a queue, is when the extension didn't actually exist. Was your invalid extension actually otherwise working and able to make and receive calls?