SpellCheck with CustomDictionaries stopped working

### Description

Hi,

SpellCheck with CustomDictionaries stopped working after an update in windows 11.

Issue also posted here https://stackoverflow.com/questions/77258504/wpf-spellcheck-on-windows-11-systems

Many thanks,

### Reproduction Steps

Create WPF app.
With abcd added to the Custom.lex

```

pack://application:,,,/Custom.lex

```

### Expected behavior

abcd to not be highlighted as a spelling error

### Actual behavior

abcd being incorrectly highlighted

### Regression?

This worked in a previous version of windows 11.

Definitely an issue on version 23H2 22631.4037
Seems to work on an old dev machine with 22H2 22621.1265

Not sure at exactly which version this stopped working.

### Known Workarounds

_No response_

### Impact

_No response_

### Configuration

Not working on net472 and net8

### Other information

_No response_