How does negative matching work in extglob in parameter expansion

Question

Problem

The behaviour of

!(pattern-list)

does not work the way I would expect when used in parameter expansion, specifically

${parameter/pattern/string}

Input

a="1 2 3 4 5 6 7 8 9 10"

Test cases

$ printf "%s
" "${a/!([0-9])/}"
[blank]
#expected 12 3 4 5 6 7 8 9 10

$ printf "%s
" "${a/!(2)/}"
[blank]
#expected  2 3 4 5 6 7 8 9 10

$ printf "%s
" "${a/!(*2*)/}"
2 3 4 5 6 7 8 9 10
#Produces the behaviour expected in previous one, not sure why though

$ printf "%s
" "${a/!(*2*)/,}"
,2 3 4 5 6 7 8 9 10
#Expected after previous worked

$ printf "%s
" "${a//!(*2*)/}"
2
#Expected again previous worked

$ printf "%s
" "${a//!(*2*)/,}"
,,2,
#Why are there 3 commas???

Specs

GNU bash, version 4.2.46(1)-release (x86_64-redhat-linux-gnu)

Notes

These are very basic examples, so if it is possible to include more complex examples with explanations in the answer then please do.

Any more info or examples needed let me know in the comments.

Have already looked at How does extglob work with shell parameter expansion?, and have even commented on what the problem is with that particular problem, so please don't mark as a dupe.