to remain true to (exp.) - to stay faithful/loyal to something or someone

Certainty/uncertainty don't work here at all. You'd need to specify what it is the designer doesn't stray from (tradition, a certain style, school of thought; the concept agreed to with the client, etc).

You can also rephrase to clarify if the designer is a purist or is simply conservative/traditional.

From https://ludwig.guru/s/remained true to the original:
"In the world of opera, when directors update repertory works to contemporary settings, they almost always remain true to the original music and libretto."

"Obviously we have to work closely with them and make sure they remain true to the original concept," Mr. Elpern said.

"Men's versions like American Eagle's Convertible Cargos, right and center ($44 at American Eagle Outfitters, South Street Seaport) and Abercrombie & Fitch's Paratrooper Convertibles, remain true to the original loose street style."

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Note added at 21 hrs (2020-07-17 14:28:01 GMT)
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Marijaflora, if you meant that the designer has her own unique way of doing things, then I wouldn't try to go for a direct translation of the term in question. I'd rephrase the sentence like you would in a marketing text so that it sells your client. Something like: "Designer X takes an eclectic approach to design, rather than adhering to traditional, and often stuffy/outdated/limiting, design philosophies." I'd talk to your client further to see what exactly it is she prides herself on and what she avoids incorporating into her work (and why) and add it in. If she insists on a word for word translation, or if this text is about, but not FOR, the actual designer, then you can say something like: "Designer X prides herself on taking an eclectic approach, rather than adhering to/remaining faithful to traditional design philosophies" to keep it more neutral.

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Note added at 22 hrs (2020-07-17 14:39:30 GMT)
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Or even: "Designer X prides herself on taking an eclectic approach, rather than adhering to/remaining faithful to/staying true to any particular style or era," since you mention her preferred modus operandi is based in eclecticism.