Special Characters { and } in Bridge Keywords

For a certain logical reason I need to enclose a particular set of keywords in some form of parentheses but not round brackets.

[ and ] cannot be use as they have special meaning in Bridge i.e. excluded keywords.

I tried < and > which work in Bridge but when exporting to Lightroom they are stripped  because Lightroom seems to interpret them as hierarchical separators.

Finally I tried { and }.

That seemd to satisfy both Bridge and Lightroom. I was happy for a while.

Then a noticed a curious occurrence - If I reimport my keywords into Bridge, keywords enclosed in the { and } characters are not imported. Their parent keyword is, but not the keywords themselves.

Does Bridge attach a special meaning to these characters which I am unaware of?