Jeremie,
The behavior you are seeing happens because of the order of normal substitutions. The inline formatting substitution (named "quotes") occurs before the attributes substitution. As a result, the inline formatting has already been applied by the time the attribute value is replaced.
One way to solve the problem is to force the substitutions to occur preemptively when the attribute value is saved. This can be accomplished using the inline pass macro around the attribute value:
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:myvar: pass:q[this "`is`" a variable]
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This is documented in the user manual here:
(There may be a better place for it and certainly it would help if it appeared in the TOC. If you have ideas, I'd love to hear them).
Another solution is to simply use smart quotes in the value. Once you learn to type them, it's actually quite a bit easier IMO.
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:myvar: this “is” a variable
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Cheers,