In Handling Bidirectional Text, we mention this:

If a formatted expression itself contains spans with differing directionality,
its formatter SHOULD perform any necessary processing, such as inserting controls or
isolating such parts to ensure that the formatted value displays correctly in a plain text context.

In the default set of functions, we should add text ensuring that they at least properly isolate their internal values. This is particularly relevant for :currency as proposed in #915 and originally discussed in #315 (comment).

Doing so might also allow for the default bidi isolation strategy to not require isolation for all placeholders in RTL messages.