Powershell: match operator returns true but $matches is null

Strictly speaking string -match ... and collection -match ... are two different operators.
The first gets a Boolean value and fills $matches.
The second gets each collection item that matches a pattern and apparently does not fill $matches.

Your example should work as you expect if the file contains a single line (the first operator works).
If a file contains 2+ lines then the second operator is used and $matches is not set.

The same is true for other Boolean operators applied to a collection.
That is collection -op ... returns items where item -op ... is true.

Examples:

1..10 -gt 5 # 6 7 8 9 10
'apple', 'banana', 'orange' -match 'e' # apple, orange 

Boolean operators applied to collections are handy if used properly.
But they may be confusing as well and lead to easy to make mistakes:

$object = @(1, $null, 2, $null)

# "not safe" comparison with $null, perhaps a mistake
if ($object -eq $null) {
    '-eq gets @($null, $null) which is evaluated to $true by if!'
}

# safe comparison with $null
if ($null -eq $object) {
    'this is not called'
}

Another example with -match and -notmatch may look confusing:

$object="apple", 'banana', 'orange'

if ($object -match 'e') {
    'this is called'
}

if ($object -notmatch 'e') {
    'this is also called, because "banana" is evaluated to $true by if!'
}