Nope, capping at 18 (high Charisma)
Working as intended...
The cap should be 21 (max charisma + 1 if wearing charisma boosting gear, if I'm not mistaken). No idea how to pass it myself and don't care.
Most I've gotten is 21 in Sanctuary. Base 6 charisma, Reginald's suit, glasses and hat for +5 more. Haven't tried any chems yet.
I'm on Xbox and I've got 2 settlements that have passed the supposed cap.
Greentop has 36
Starlight has 29
I'm on Xbox and have several over 30, including 3 over 40 (Kingsport, the Drive In & the Co-op) and one (The Castle) with 99 as my population cap didn't seem to click in
Note: My game crashes every time I fast travel straight to Castle
38 at Greentop Nursery - I hadn't travelled there for some time and when I did I found the population had spiked. Only had 4 beds too so some interesting sleeping arrangements snigger snigger
Edit: XBox One
Not from what I have seen. they stay there anyway.