In T8984#151155, @ltoscano wrote:

Do you mean that those markers are needed anytime there is a mixed string with a translated part (which could be LTR or RTL) and a fixed part which has always the same direction?

Yes I mean that. In general, whenever the final string can have both RTL and LTR texts, enclosing one of them (non-strong) is a must.

In T8984#151155, @ltoscano wrote:

Can you please show how it would work? Would those placeholders always be added to the appropriate string? Wouldn't it be possible to always inject them through i18n functions?

This is just a fast idea: we have "%s".arg(part), we can just enclose that %s with the markers to be "FSI%sPDI".arg(part), where FSI and PDI are real Unicode characters.
I had to share my initial thoughts about this, but I have no big experience with coding, neither with KI18N code database. I can't work on this on the near future, but if no one took this task, I'll try my best implementing it myself later.